Strengthened glass with deep depth of compression

ABSTRACT

Chemically strengthened glass articles having at least one deep compressive layer extending from a surface of the article to a depth of at least about 45 μm within the article are provided. In one embodiment, the compressive stress profile includes a single linear segment extending from the surface to the depth of compression DOC. Alternatively, the compressive stress profile includes two linear portions: the first portion extending from the surface to a relatively shallow depth and having a steep slope; and a second portion extending from the shallow depth to the depth of compression. The strengthened glass has a 60% survival rate when dropped from a height of 80 cm in an inverted ball drop test and an equibiaxial flexural strength of at least 10 kgf as determined by abraded ring-on-ring testing. Methods of achieving such stress profiles are also described.

This application claims the benefit of priority under 35 U.S.C. §119 ofU.S. Provisional Patent Application Ser. No. 61/943,758, filed on Feb.24, 2014; U.S. Provisional Patent Application Ser. No. 62/014,464, filedon Jun. 19, 2014; U.S. Provisional Patent Application Ser. No.62/014,372, filed on Jun. 19, 2014; and U.S. Provisional PatentApplication Ser. No. 62/029,075, filed on Jul. 25, 2014, the contents ofwhich are relied upon and incorporated herein by reference in itsentirety.

BACKGROUND

The disclosure relates to a chemically strengthened glass article. Moreparticularly, the disclosure relates to chemically strengthened glasseshaving a deep compressive surface layer.

Strengthened glasses are widely used in electronic devices as coverplates or windows for portable or mobile electronic communication andentertainment devices, such as cellular phones, smart phones, tablets,video players, information terminal (IT) devices, laptop computers andthe like, as well as in other applications. As strengthened glasses areincreasingly being utilized, it has become more important to developstrengthened glass materials having improved survivability, especiallywhen subjected to tensile stresses and/or relatively deep flaws causedby contact with hard/sharp surfaces.

SUMMARY

Chemically strengthened glass articles having at least one deepcompressive layer extending from a surface of the article to a depth ofat least about 45 μm within the article are provided. In one embodiment,the compressive stress profile includes a single linear segment orportion extending from the surface to the depth of compression DOC.Alternatively, the compressive stress profile includes two approximatelylinear portions: the first portion extending from the surface to arelatively shallow depth and having a relatively steep slope; and asecond portion extending from the shallow depth to the depth ofcompression. The strengthened glass has a 60% survival rate when droppedfrom a height of 100 cm in an inverted ball drop test and an equibiaxialflexural strength of at least 10 kgf as determined by abradedring-on-ring testing. Methods of achieving such stress profiles are alsodescribed.

Accordingly, one aspect of the disclosure is to provide a glass article.The glass article has a thickness t and a compressive layer having asurface compressive stress CS in a range from about 700 MPa up to about1000 MPa at a surface. The compressive layer extends from the surface toa depth of compression DOC, wherein DOC≧0.1·t when t<0.5 mm and DOC≧50μm when t≧0.5 mm. The compressive layer has a compressive stress profilecomprising a first portion b extending from the surface to a depth d_(b)and having a slope m_(b), wherein 3 μm≦d_(b)≦15 μm and −40MPa/μm≧m_(b)≧−200 MPa/μm; and a second portion c extending from d_(b) tothe depth of compression DOC and having a slope m_(c), wherein −2MPa/μm≧m_(c)≧−8 MPa/μm.

In a second aspect, a glass article is provided. The glass article has athickness t and a compressive region having a compressive stress CS in arange from about 150 MPa to about 400 MPa at a surface. The compressiveregion extends from the surface to a depth of compression DOC and has acompressive stress profile, wherein DOC≧0.1·t when t<0.5 mm and DOC≧50μm when t≧0.5 mm. The compressive stress profile has a linear portion aextending from the surface to a depth d_(a) and a slope m_(a), whereinthe depth d_(a) is equal to the depth of compression and −2MPa/μm≧m_(a)≧−8 MPa/μm.

A third aspect of the disclosure is to provide a strengthened glass. Thestrengthened glass has a thickness t, an inner region under a centraltension CT, and at least one compressive layer under a compressivestress CS at a surface. The compressive layer extends from the surfaceof the glass to a depth of compression DOC, wherein DOC≧0.1·t when t<0.5mm and DOC≧50 μm when t≧0.5 mm, and is adjacent to the inner region. Thestrengthened glass has at least a 60% survival rate when dropped in aninverted ball drop test from a height of about 80 cm onto an abrasivematerial disposed on the surface of the glass.

A fourth aspect of the disclosure is to provide a strengthened glass.The strengthened glass the strengthened glass has a thickness t, aninner region under a central tension CT, and at least one compressivestress layer under a compressive stress CS at a surface of the glass.The compressive stress layer extends from a surface of the glass to adepth of compression DOC, wherein DOC≧0.1·t when t<0.5 mm and DOC≧50 μmwhen t≧0.5 mm, and is adjacent to the inner region. The strengthenedglass has an equibiaxial flexural strength of at least 10 kgf up toabout 50 kgf as determined by abraded ring-on-ring testing.

These and other aspects, advantages, and salient features will becomeapparent from the following detailed description, the accompanyingdrawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a chemically strengthenedglass article;

FIG. 2 is a schematic representation of a compressive stress profileobtained by a single step ion exchange process;

FIG. 3 is a schematic representation of a compressive stress profileobtained by a two-step ion exchange process;

FIG. 4a is a plot of spectra of refractive index profiles for TM and TEpolarization reconstructed form the respective TM and TE spectra ofbound optical modes measured for ion exchanged glass sample a having athickness of 0.4 mm;

FIG. 4b is a plot of the compressive stress profile determined from theindex profiles shown in FIG. 4 a;

FIG. 5a is a plot of TM and TE refractive index profiles reconstructedfrom spectra of bound optical modes for TM and TE polarization measuredfor ion exchanged glass sample b having a thickness of 0.4 mm;

FIG. 5b is a plot of the compressive stress profile determined from theindex profiles shown in FIG. 5 a;

FIG. 5c is a plot of the compressive stress profile of the sample inFIGS. 5a and 5b following a second ion exchange step;

FIG. 6a is a plot of TM and TE refractive index profiles reconstructedfrom spectra of bound optical modes for TM and TE polarization measuredfor ion exchanged glass sample c having a thickness of 0.4 mm;

FIG. 6b is a plot of the compressive stress profile determined from theindex profiles shown in FIG. 6 a;

FIG. 7a is a plot of TM and TE refractive index profiles reconstructedfrom spectra of bound optical modes for TM and TE polarization measuredfor ion exchanged glass sample d having a thickness of 0.5 mm;

FIG. 7b is a plot of the compressive stress profile determined from theindex profiles shown in FIG. 7 a;

FIG. 8a is a plot of TM and TE refractive index profiles reconstructedfrom spectra of bound optical modes for TM and TE polarization measuredfor ion exchanged glass sample e having a thickness of 0.5 mm;

FIG. 8b is a plot of the compressive stress profile determined from theindex profiles shown in FIG. 8 a;

FIG. 9a is a plot of the compressive stress profile for ion exchangedglass sample f having a thickness of 0.7 mm;

FIG. 9b is a plot of the compressive stress profile of the sample inFIG. 9a following a second ion exchange step;

FIG. 10a is a plot of the compressive stress profile for ion exchangedglass sample g having a thickness of 0.8 mm;

FIG. 10b is a plot of the compressive stress profile of the sample inFIG. 10a following a second ion exchange step;

FIG. 11 is a plot of the compressive stress profile for ion exchangedglass sample i having a thickness of 0.9 mm following two ion exchangesteps;

FIG. 12a is a plot of the compressive stress profile for ion exchangedglass sample j having a thickness of 1.0 mm;

FIG. 12b is a plot of the compressive stress profile determined for thesample of FIG. 12a following a second ion exchange step;

FIG. 13a is a plot of the compressive stress profile for ion exchangedglass sample k having a thickness of 0.55 mm;

FIG. 13b is a plot of the compressive stress profile determined for thesample of FIG. 13a following a second ion exchange step;

FIG. 14a is a graphical representation of a photograph showingstrengthened glass articles 1) exhibiting frangible behavior uponfragmentation; and 2) exhibiting non-frangible behavior uponfragmentation;

FIG. 14b is a graphical representation of a photograph showingstrengthened glass sheets that exhibit non-frangible behavior uponfragmentation;

FIG. 15a is a schematic cross-sectional view of prior art apparatus thatis used to perform ball drop testing;

FIG. 15b is a schematic cross-sectional view of an embodiment of theapparatus that is used to perform the inverted ball on sandpaper (IBoS)test described in the present disclosure;

FIG. 15c is a schematic cross-sectional representation of the dominantmechanism for failure due to damage introduction plus bending thattypically occurs in strengthened glass articles that are used in mobileor hand held electronic devices;

FIG. 15d is a flow chart for a method of conducting the IBoS test in theapparatus described herein;

FIG. 16 is a graphical comparison of failure rates of strengthenedglasses at varying DOL and CS values when subjected to the IBoS testdescribed in the present disclosure;

FIG. 17 is a plot of the compressive stress profile for ion exchangedglass sample m having a thickness of 0.8 mm;

FIG. 18a is a plot of drop height at failure as a function of depth oflayer DOL, as measured by FSM, of ion exchanged glass samples;

FIG. 18b is a plot of average failure height as a function of glasssample thickness;

FIG. 19 is a schematic cross-sectional view of a ring-on-ring apparatus;and

FIG. 20 is a plot of abraded ring-on-ring data as a function of samplethickness for two strengthened alkali aluminosilicate glasses.

DETAILED DESCRIPTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views shown in thefigures. It is also understood that, unless otherwise specified, termssuch as “top,” “bottom,” “outward,” “inward,” and the like are words ofconvenience and are not to be construed as limiting terms. In addition,whenever a group is described as comprising at least one of a group ofelements and combinations thereof, it is understood that the group maycomprise, consist essentially of, or consist of any number of thoseelements recited, either individually or in combination with each other.Similarly, whenever a group is described as consisting of at least oneof a group of elements or combinations thereof, it is understood thatthe group may consist of any number of those elements recited, eitherindividually or in combination with each other. Unless otherwisespecified, a range of values, when recited, includes both the upper andlower limits of the range as well as any ranges therebetween. As usedherein, the indefinite articles “a,” “an,” and the correspondingdefinite article “the” mean “at least one” or “one or more,” unlessotherwise specified. It also is understood that the various featuresdisclosed in the specification and the drawings can be used in any andall combinations.

As used herein, the terms “glass article” and “glass articles” are usedin their broadest sense to include any object made wholly or partly ofglass. Unless otherwise specified, all glass compositions are expressedin terms of mole percent (mol %) and all ion exchange bath compositionsare expressed in terms of weight percent (wt %).

It is noted that the terms “substantially” and “about” may be utilizedherein to represent the inherent degree of uncertainty that may beattributed to any quantitative comparison, value, measurement, or otherrepresentation. These terms are also utilized herein to represent thedegree by which a quantitative representation may vary from a statedreference without resulting in a change in the basic function of thesubject matter at issue. Thus, a glass that is “substantially free ofMgO” is one in which MgO is not actively added or batched into theglass, but may be present in very small amounts as a contaminant; i.e.,less than 0.1 mol %.

Referring to the drawings in general and to FIG. 1 in particular, itwill be understood that the illustrations are for the purpose ofdescribing particular embodiments and are not intended to limit thedisclosure or appended claims thereto. The drawings are not necessarilyto scale, and certain features and certain views of the drawings may beshown exaggerated in scale or in schematic in the interest of clarityand conciseness.

As used herein, the terms “depth of layer” and “DOL” refer to the depthof the compressive layer as determined by surface stress meter (FSM)measurements using commercially available instruments such as theFSM-6000.

As used herein, the terms “depth of compression” and “DOC” refer to thedepth at which the stress within the glass changes from compressive totensile stress. At the DOC, the stress crosses from a positive(compressive) stress to a negative (tensile) stress and thus has a valueof zero.

As described herein, compressive stress (CS) and central tension (CT)are expressed in terms of megaPascals (MPa), depth of layer (DOL) anddepth of compression (DOC) are expressed in terms of microns (μm), where1 μm=0.001 mm, and thickness t is expressed herein in terms ofmillimeters, where 1 mm=1000 μm, unless otherwise specified.

As used herein, the term “fracture,” unless otherwise specified, meansthat a crack propagates across the entire thickness and/or entiresurface of a substrate when that substrate is dropped or impacted withan object.

According to the scientific convention normally used in the art,compression is expressed as a negative (<0) stress and tension isexpressed as a positive (>0) stress. Throughout this description,however, compressive stress CS is expressed as a positive or absolutevalue—i.e., as recited herein, CS=|CS| and central tension or tensilestress is expressed as a negative value in order to better visualize thecompressive stress profiles described herein.

As used herein, the “slope (m)” refers to the slope of a segment orportion of the stress profile that closely approximates a straight line.The predominant slope is defined as the average slope for regions thatare well approximated as straight segments. These are regions in whichthe absolute value of the second derivative of the stress profile issmaller than the ratio of the absolute value of the first derivative,and approximately half the depth of the region. For a steep, shallowsegment of the stress profile near the surface of the strengthened glassarticle, for example, the essentially straight segment is the portionfor each point of which the absolute value of the second derivative ofthe stress profile is smaller than the absolute value of the local slopeof the stress profile divided by the depth at which the absolute valueof the stress changes by a factor of 2. Similarly, for a segment of theprofile deeper within the glass, the straight portion of the segment isthe region for which the local second derivative of the stress profilehas an absolute value that is smaller than the absolute value of thelocal slope of the stress profile divided by half the DOC.

For typical stress profiles, this limit on the second derivativeguarantees that the slope changes relatively slowly with depth, and istherefore reasonably well defined and can be used to define regions ofslope that are important for the stress profiles that are consideredadvantageous for drop performance.

Let the stress as profile a function of depth x be given by the functionσ=σ(x)  (1),and let the first derivative of the stress profile with respect to depthbe

$\begin{matrix}{{\sigma^{\prime} = \frac{\mathbb{d}\sigma}{\mathbb{d}x}},} & (2)\end{matrix}$and the second derivative be

$\begin{matrix}{\sigma^{''} = {\frac{\mathbb{d}^{2}\sigma}{\mathbb{d}x^{2}}.}} & (3)\end{matrix}$

If a shallow segment extends approximately to a depth d_(s), then forthe purposes of defining a predominant slope, a straight portion of theprofile is a region where

$\begin{matrix}{{{\sigma^{''}(x)}} < {{{2\frac{\sigma^{\prime}(x)}{d_{s}}}}.}} & (4)\end{matrix}$

If a deep segment extends approximately to a larger depth DOC, or to alarger depth d_(d), or to a depth DOL in traditional terms, then astraight portion of the profile is a region where

$\begin{matrix}{{{\sigma^{''}(x)}} < {{2\frac{\sigma^{\prime}(x)}{d_{d}}}} \approx {{2\frac{\sigma^{\prime}(x)}{DOC}}} \approx {{{2\frac{\sigma^{\prime}(x)}{DOL}}}.}} & (5)\end{matrix}$

The latter equation is also valid for a 1-segment stress profileobtained by a single ion exchange in a salt containing only a singlealkali ion other than the ion being replaced in the glass for chemicalstrengthening.

Preferably, the straight segments are selected as regions where

$\begin{matrix}{{{{\sigma^{''}(x)}} < {\frac{\sigma^{\prime}(x)}{d}}},} & (6)\end{matrix}$where d stands for the relevant depth for the region, shallow or deep.

The slope m of linear segments of the compressive stress profilesdescribed herein are given as absolute values of the slope

${\frac{\mathbb{d}\sigma}{\mathbb{d}x} - {i.e.}},$m, as recited herein, is equal to

${\frac{\mathbb{d}\sigma}{\mathbb{d}x}}.$More specifically, the slope m represents the absolute value of theslope of a profile for which the compressive stress generally decreasesas a function of increasing depth.

Described herein are glass articles that are chemically strengthened byion exchange to obtain a prescribed compressive stress profile and thusachieve survivability when dropped onto a hard, abrasive surface from aprescribed height.

Compressive stress CS and depth of layer DOL are stress profileparameters that have been used for years to enable quality control ofchemical strengthening. Compressive stress CS provides an estimate ofthe surface compression, an important parameter that correlates wellwith the amount of stress that needs to be applied to cause a failure ofa glass article, particularly when the glass is free of substantiallydeep mechanical flaws. Depth of layer DOL has been used as anapproximate measure of the depth of penetration of the larger(strengthening) cation (e.g., K⁺ during K⁺ for Na⁺ exchange), withlarger DOL correlating well with greater depths of the compressionlayer, protecting the glass by arresting deeper flaws, and preventingflaws from causing failure under conditions of relatively low externallyapplied stress.

Even with minor to moderate bending of a glass article, the bendingmoment induces a stress distribution that is generally linear with depthfrom the surface, having a maximum tensile stress on the outer side ofbending, a maximum compressive stress on the inner side of the bending,and zero stress at the so-called neutral surface, which is usually inthe interior. For tempered glass parts, this bending-inducedconstant-slope stress distribution is added to the tempering stressprofile to result in the net stress profile in the presence of external(bending) stress.

The net profile in the presence of bending-induced stress generally hasa different depth of compression DOC from the stress profile withoutbending. In particular, on the outer side of bending, the depth ofcompression DOC is reduced in the presence of bending. If the temperingstress profile has a relatively small stress slope at depths in thevicinity of and smaller than the DOC, the DOC can drop verysubstantially in the presence of bending. In the net stress profile, thetips of moderately deep flaws could be exposed to tension, while thesame flaw tips would normally be arrested in the compression region ofthe tempering profile without bending. These moderately deep flaws canthus grow and lead to fracture during the bending.

Bending stresses are also important during drop testing. Regions oflocalized time-varying stress occur during mechanical vibrations andwave propagation through the glass article. With increasing drop height,the glass article experiences higher time-varying stresses duringcontact with the floor surface as well as during vibrations occurringafter contact. Thus, some fracture failures may occur due to excessivepost-contact tensile stress occurring at the tip of a relatively shallowflaw that would normally be innocuous in the presence of temperingwithout these time-varying stresses.

The present disclosure describes a range of slopes that provides a goodtrade-off between performance of the glass article during drop tests andduring bending tests. The preferable ranges may in some cases bepartially defined or limited by the capabilities and limitations ofstress measurement equipment (such as, for example, the FSM-6000 stressmeter) for collection and interpretation of spectra associated withthese profiles for the purposes of quality control during production.Not only the depth of layer DOL, but also the slope of the stressprofile (through the slope of the index profile associated with thestress profile), affect the ability to resolve particular lines in thecoupling spectra, and thus to control product quality effectively.

Ion exchange is commonly used to chemically strengthen glasses. In oneparticular example, alkali cations within a source of such cations(e.g., a molten salt, or “ion exchange,” bath) are exchanged withsmaller alkali cations within the glass to achieve a layer that is undera compressive stress (CS) near the surface of the glass. For example,potassium ions from the cation source are often exchanged with sodiumions within the glass. The compressive layer extends from the surface toa depth within the glass.

A cross-sectional schematic view of a planar ion exchanged glass articleis shown in FIG. 1. Glass article 100 has a thickness t, first surface110, and second surface 112. In some embodiments, glass article 100 hasa thickness t of at least 0.15 mm and up to about (i.e., less than orequal to) about 2.0 mm, or up to about 1.0 mm, or up to about 0.7 mm, orup to about 0.5 mm. While the embodiment shown in FIG. 1 depicts glassarticle 100 as a flat planar sheet or plate, glass article 100 may haveother configurations, such as a three dimensional shape or anothernon-planar configuration. Glass article 100 has a first compressiveregion 120 extending from first surface 110 to a depth of compression(DOC) d₁ into the bulk of the glass article 100. In the embodiment shownin FIG. 1, glass article 100 also has a second compressive region 122extending from second surface 112 to a second depth of compression (DOC)d₂. Glass article 100 also has a central region 130 that extends from d₁to d₂. Central region 130 is under a tensile stress, having a maximumvalue at the center of the central region 130, referred to as centraltension or center tension (CT). The tensile stress of region 130balances or counteracts the compressive stresses of regions 120 and 122.The depths d₁, d₂ of first and second compressive regions 120, 122protect the glass article 100 from the propagation of flaws introducedby sharp impact to first and/or second surfaces 110, 112 of glassarticle 100, while the compressive stress CS minimizes the likelihood ofa flaw growing and penetrating through the depth d₁, d₂ of first andsecond compressive regions 120, 122.

The strengthened glass articles described herein have a maximumcompressive stress CS of at least about 150 megaPascals (MPa) up toabout 400 MPa, or up to about 1000 MPa. In some embodiments, the maximumcompressive stress CS is at least about 210 MPa and, in otherembodiments, at least about 300 MPa. In some embodiments, the maximumcompressive stress CS is located at the surface (110, 112 in FIG. 1). Inother embodiments, however, the maximum compressive CS may be located inthe compressive region (120, 122) at some depth below the surface of theglass article. Each compressive region (120, 122) extends from thesurface of the glass article to a depth of compression DOC (d₁, d₂) ofat least about 50 microns (μm) up to about 100 μm. In some embodiments,DOC is at least about 60 μm. In other embodiments, DOC is at least about70 μm, in some embodiments, at least about 80 μm, and, in still otherembodiments, DOC is at least about 90 μm. In certain embodiments, thedepth of compression DOC has a maximum value of about 100 μm. The depthof compression DOC (d₁, d₂) may also be expressed in terms of thethickness t of the glass article 100. In some embodiments,0.1·t≦DOC≦0.2·t when t<0.5 mm and, in other embodiments, DOC≧50 μm whent≧0.5 mm.

The compressive stress varies as a function of depth below the surfaceof the strengthened glass article, producing a compressive stressprofile in the compressive region. In some embodiments, the compressivestress profile is substantially linear from the surface to the depth ofcompression DOC, as schematically shown in FIG. 2. As seen in FIG. 2,the compressive stress profile a is linear with respect to depth belowthe surface, resulting in a straight line having a slope m_(a),expressed in MPa/μm that intercepts the vertical y (CS) axis at CS_(s).CS profile a intercepts the x axis at depth d_(a), which is the depth ofcompression DOC. At this point, the total stress (tension+compression)is zero. Below DOC, the glass article is in tension CT, reaching acentral value CT. In one non-limiting example, there may be a sub-regionover which the tension varies from 0 up to a maximum (by absolute value)tension equal to CT, and a region over which the tension issubstantially constant and equal to CT.

In some embodiments, the compressive stress profile a of the glassarticle described herein has a slope m_(a) that is within a specifiedrange. In FIG. 2, for example, slope m_(a) of line a lies between upperboundary δ₁ and lower boundary δ₂; i.e., δ₁≧m_(a)≧δ₂. In someembodiments, −2 MPa/μm≧m_(a)≧−200 MPa/μm. In some embodiments, −2MPa/μm≧m_(a)≧−8 MPa/μm, in some embodiments, −3 MPa/μm≧m_(a)≧−6 MPa/μm,and in still other embodiments, −2 MPa/μm≧m_(a)≧−4.5 MPa/μm.

In other embodiments, the compressive stress profile may comprise two ormore substantially linear functions, as schematically shown in FIG. 3.As seen in FIG. 3, the compressive stress profile has a first segment orportion b and a second segment or portion c. First portion b exhibitssubstantially linear behavior from the strengthened surface of the glassarticle to a depth d_(b). Portion b has a slope m_(b) and y interceptCS_(s). Second segment or portion c of the compressive stress profileextends from approximately depth d_(b) to the depth of compression DOC,and has a slope m_(c). The compressive stress CS(d_(b)) at depth d_(b)is given by the expressionCS(d _(b))≈CS_(s) −d _(b)(m _(b))  (7).In some embodiments, depth d_(b) is in a range from about 3 μm to about8 μm; i.e., 3 μm≧d_(b)≧15 μm. In other embodiments, 3 μm≦d_(b)≦10 μm. Instill other embodiments, 3 μm≦d_(b)≦12 μm.

It will be appreciated by those skilled in the art that the presentdisclosure is not limited to compressive stress profiles consisting ofonly two distinct portions. Instead, the compressive stress profile mayinclude additional segments. In some embodiments, different linearportions or segments of the compressive stress profile may be joined bya transitional region (not shown) in which the slope of the profiletransitions from a first slope to a second slope (e.g., from m_(b) tom_(c)).

As shown in FIG. 3, the slope of portion b of the compressive stressprofile is much steeper than the slope of portion c; i.e.,|m_(b)|≧|m_(c)|. This corresponds to a condition in which a compressivestress profile having a “spike” at the surface of the glass article iscreated by multiple ion exchange processes carried out in succession inorder to provide the surface with sufficient compressive stress towithstand the introduction or growth of some flaws produced throughimpact.

In some embodiments, the compressive stress profiles b and c of theglass article described herein have slopes m_(b) and m_(c),respectively, that are within specified ranges. In FIG. 3, for example,slope m_(b) of line/first portion b lies between upper boundary δ₃ andlower boundary δ₄ and slope m_(c) of line/second portion c lies betweenupper boundary δ₅ and lower boundary δ₆; i.e., δ₃≧m_(b)≧δ₄ andδ₅≧m_(c)≧δ₆. In some embodiments, −40 MPa/μm≧m_(b)≧−200 MPa/μm, and −2MPa/μm≧m_(c)≧−8 MPa/μm. In some embodiments, −40 MPa/μm≧m_(b)≧−120MPa/μm and, in still other embodiments, −50 MPa/μm≧m_(b)≧−120 MPa/μm.

Compressive stress CS and depth of the compressive layer (referred to as“depth of layer” or DOL) are measured using those means known in theart. Such means include, but are not limited to, measurement of surfacestress (FSM) using commercially available instruments such as theFSM-6000, manufactured by Luceo Co., Ltd. (Tokyo, Japan), or the like.Methods of measuring compressive stress and depth of layer are describedin ASTM 1422C-99, entitled “Standard Specification for ChemicallyStrengthened Flat Glass,” and ASTM 1279.19779 “Standard Test Method forNon-Destructive Photoelastic Measurement of Edge and Surface Stresses inAnnealed, Heat-Strengthened, and Fully-Tempered Flat Glass,” thecontents of which are incorporated herein by reference in theirentirety. Surface stress measurements rely upon the accurate measurementof the stress optical coefficient, which is related to the birefringenceof the glass. The stress optical coefficient in turn is measured bythose methods that are known in the art, such as fiber and four pointbend methods, both of which are described in ASTM standard C770-98(2008), entitled “Standard Test Method for Measurement of GlassStress-Optical Coefficient,” the contents of which are incorporatedherein by reference in their entirety, and a bulk cylinder method.

The relationship between CS and central tension CT may, in someembodiments, be approximated by the expression:CT=(CS·DOL)/(t−2DOL)  (8),where t is the thickness, expressed in microns (μm), of the glassarticle. In various sections of the disclosure, central tension CT andcompressive stress CS are expressed herein in megaPascals (MPa),thickness t is expressed in either microns (μm) or millimeters (mm), anddepth of layer DOL is expressed in microns (μm) or millimeters (mm),consistent with the representation of t.

For strengthened glass articles in which the compressive stress layersextend to deeper depths within the glass, the FSM technique may sufferfrom contrast issues which affect the observed DOL value. At deeper DOLvalues, there may be inadequate contrast between the TE and TM spectra,thus making the calculation of the difference between TE and TMspectra—and thus determining the DOL—more difficult. Moreover, the FSMsoftware analysis is incapable of determining the compressive stressprofile (i.e., the variation of compressive stress as a function ofdepth within the glass). In addition, the FSM technique is incapable ofdetermining the depth of layer resulting from the ion exchange ofcertain elements such as, for example, ion exchange of sodium forlithium.

The DOL as determined by the FSM is a relatively good approximation forthe depth of compression (DOC) when the DOL is a small fraction r of thethickness t and the index profile has a depth distribution with isreasonably well approximated with a simple linear truncated profile.When the DOL is a substantial fraction of the thickness, such asDOL≧0.1·t, then the DOC is most often noticeably lower than the DOL. Forexample, in the idealized case of a linear truncated profile, therelationship DOC=DOL (1−r) holds, where r=DOL/t.

Most TM and TE index profiles have a curved portion near the bottom ofthe index profile, and the relationship between DOC and DOL then may besomewhat more involved, but generally the ratio DOC/DOL decreases as rincreases. For some profile shapes it is possible to have even DOC≧DOL,particularly when r<0.02.

When the concentration profile of the larger (strengthening) cation(e.g., K⁺) introduced by ion exchange has two segments, with the segmentone nearest the surface having a substantially higher concentration, andthe segment spread over large depths and having a substantially lowerconcentration, the DOL as found by the FSM is significantly smaller thanthe overall depth of chemical penetration of the larger ion. This is incontrast with the case of a simple one-segment diffusion profile inwhich the DOL provides a good estimate of the chemical penetration. In atwo-segment profile, the DOC may be larger or smaller than the DOL,depending on the depth and stress parameters of the profile and on thethickness.

When low external stresses are applied to a strengthened glass, thefracture-causing flaws have depths that correlate better with the DOCrather than the DOL. The reason why DOL has been used successfully as ahigh-value parameter of chemical strengthening is that for simplesingle-segment stress profiles, the DOL has had a good correlation withDOC. In addition, the DOC and the DOL have been similar, since for manyyears the DOL has been generally lower than 0.1·t, and for the most partlower than 0.05·t. Thus, for traditional chemically-strengthened glass,the DOL has had good correlation with the depth of strength-limitingflaws.

With the increasing importance of thinner cover glasses (e.g., havingt<0.5 mm) and the introduction of deeper and more complex stressprofiles aimed at improving drop performance while preserving highstrength under high-stress tests such as ring-on-ring (ROR), abradedring-on-ring (AROR), and four-point-bend (4PB), the depth of layer DOLdeviates significantly from the depth of compression DOC.Fracture-inducing flaws under conditions of low external stress oftenoccur at depths smaller than the DOL, but are consistent with the DOC.

The techniques described below have been developed to more accuratelydetermine the depth of compression (DOC) and compressive stress profilesfor strengthened glass articles.

Two methods for extracting detailed and precise stress profiles (stressas a function of depth) of tempered or chemically strengthened glass aredisclosed in U.S. patent application Ser. No. 13/463,322, entitled“Systems And Methods for Measuring the Stress Profile of Ion-ExchangedGlass (hereinafter referred to as “Roussev I”),” filed by Rostislav V.Roussev et al. on May 3, 2012, and claiming priority to U.S. ProvisionalPatent Application No. 61/489,800, having the same title and filed onMay 25, 2011. The spectra of bound optical modes for TM and TEpolarization are collected via prism coupling techniques and used intheir entirety to obtain detailed and precise TM and TE refractive indexprofiles n_(TM)(z) and n_(TE)(z). In one embodiment, the detailed indexprofiles are obtained from the mode spectra by using the inverseWentzel-Kramers-Brillouin (IWKB) method. The contents of the abovepatent applications are incorporated herein by reference in theirentirety.

In another embodiment, the detailed index profiles are obtained byfitting the measured mode spectra to numerically calculated spectra ofpre-defined functional forms that describe the shapes of the indexprofiles and obtaining the parameters of the functional forms from thebest fit. The detailed stress profile S(z) is calculated from thedifference of the recovered TM and TE index profiles by using a knownvalue of the stress-optic coefficient (SOC):S(z)=[n _(TM)(z)−n _(TE)(z)]/SOC  (9).

Due to the small value of the SOC, the birefringence n_(TM)(z)−n_(TE)(z)at any depth z is a relatively small fraction (typically on the order of1%) of either of the indices n_(TM)(z) and n_(TE)(z). Obtaining stressprofiles that are not significantly distorted due to noise in themeasured mode spectra requires determination of the mode effectiveindices with precision on the order of 0.00001 RIU (refractive indexunits). The methods disclosed in Roussev I further include techniquesapplied to the raw data to ensure such high precision for the measuredmode indices, despite noise and/or poor contrast in the collected TE andTM mode spectra or images of the mode spectra. Such techniques includenoise-averaging, filtering, and curve fitting to find the positions ofthe extremes corresponding to the modes with sub-pixel resolution.

Similarly, U.S. patent application Ser. No. 14/033,954, entitled“Systems and Methods for Measuring Birefringence in Glass andGlass-Ceramics (hereinafter “Roussev II”),” filed by Rostislav V.Roussev et al. on Sep. 23, 2013, and claiming priority to U.S.Provisional Application Ser. No. 61/706,891, having the same title andfiled on Sep. 28, 2012, discloses apparatus and methods for opticallymeasuring birefringence on the surface of glass and glass ceramics,including opaque glass and glass ceramics. Unlike Roussev I, in whichdiscrete spectra of modes are identified, the methods disclosed inRoussev II rely on careful analysis of the angular intensitydistribution for TM and TE light reflected by a prism-sample interfacein a prism-coupling configuration of measurements. The contents of theabove patent applications are incorporated herein by reference in theirentirety.

In another disclosed method, derivatives of the TM and TE signals aredetermined after application of some combination of the aforementionedsignal conditioning techniques. The locations of the maximum derivativesof the TM and TE signals are obtained with sub-pixel resolution, and thesurface birefringence is proportional to the spacing of the above twomaxima, with a coefficient determined as before by the apparatusparameters.

Associated with the requirement for correct intensity extraction, theapparatus comprises several enhancements, such as using alight-scattering surface (static diffuser) in close proximity to or onthe prism entrance surface to improve the angular uniformity ofillumination, a moving diffuser for speckle reduction when the lightsource is coherent or partially coherent, and light-absorbing coatingson portions of the input and output facets of the prism and on the sidefacets of the prism, to reduce parasitic background which tends todistort the intensity signal. In addition, the apparatus may include aninfrared light source to enable measurement of opaque materials.

Furthermore, Roussev II discloses a range of wavelengths and attenuationcoefficients of the studied sample, where measurements are enabled bythe described methods and apparatus enhancements. The range is definedby α_(s)λ<250πσ_(s), where α_(s) is the optical attenuation coefficientat measurement wavelength λ, and σ_(s) is the expected value of thestress to be measured with typically required precision for practicalapplications. This wide range allows measurements of practicalimportance to be obtained at wavelengths where the large opticalattenuation renders previously existing measurement methodsinapplicable. For example, Roussev II discloses successful measurementsof stress-induced birefringence of opaque white glass-ceramic at awavelength of 1550 nm, where the attenuation is greater than about 30dB/mm.

While it is noted above that there are some issues with the FSMtechnique at deeper DOL values, FSM is still a beneficial conventionaltechnique which may utilized with the understanding that an error rangeof up to +/−20% is possible at deeper DOL values. The terms “depth oflayer” and “DOL” as used herein refer to DOL values computed using theFSM technique, whereas the terms “depth of compression” and “DOC” referto depths of the compressive layer determined by the methods describedin Roussev I & II.

As stated above, the glass articles may be chemically strengthened byion exchange. In this process, ions at or near the surface of the glassare replaced by—or exchanged with—larger ions usually having the samevalence or oxidation state. In those embodiments in which the glassarticle comprises, consists essentially of, or consists of an alkalialuminosilicate glass, ions in the surface layer of the glass and thelarger ions are monovalent alkali metal cations, such as Na⁺ (when Li⁺is present in the glass), K⁺, Rb⁺, and Cs⁺. Alternatively, monovalentcations in the surface layer may be replaced with monovalent cationsother than alkali metal cations, such as Ag⁺ or the like.

Ion exchange processes are typically carried out by immersing a glassarticle in a molten salt bath containing the larger ions to be exchangedwith the smaller ions in the glass. It will be appreciated by thoseskilled in the art that parameters for the ion exchange process,including, but not limited to, bath composition and temperature,immersion time, the number of immersions of the glass in a salt bath (orbaths), use of multiple salt baths, and additional steps such asannealing, washing and the like, are generally determined by thecomposition of the glass and the desired depth of layer and compressivestress of the glass that result from the strengthening operation. By wayof example, ion exchange of alkali metal-containing glasses may beachieved by immersion in at least one molten bath containing a salt suchas, but not limited to, nitrates, sulfates, and chlorides of the largeralkali metal ion. The temperature of the molten salt bath typically isin a range from about 380° C. up to about 450° C., while immersion timesrange from about 15 minutes up to about 40 hours. However, temperaturesand immersion times different from those described above may also beused.

In addition, non-limiting examples of ion exchange processes in whichglass is immersed in multiple ion exchange baths, with washing and/orannealing steps between immersions, are described in U.S. Pat. No.8,561,429, by Douglas C. Allan et al., issued on Oct. 22, 2013, entitled“Glass with Compressive Surface for Consumer Applications,” and claimingpriority from U.S. Provisional Patent Application No. 61/079,995, filedJul. 11, 2008, in which glass is strengthened by immersion in multiple,successive, ion exchange treatments in salt baths of differentconcentrations; and U.S. Pat. No. 8,312,739, by Christopher M. Lee etal., issued on Nov. 20, 2012, and entitled “Dual Stage Ion Exchange forChemical Strengthening of Glass,” and claiming priority from U.S.Provisional Patent Application No. 61/084,398, filed Jul. 29, 2008, inwhich glass is strengthened by ion exchange in a first bath is dilutedwith an effluent ion, followed by immersion in a second bath having asmaller concentration of the effluent ion than the first bath. Thecontents of U.S. Pat. Nos. 8,561,429 and 8,312,739 are incorporatedherein by reference in their entirety.

The compressive stress is created by chemically strengthening the glassarticle, for example, by the ion exchange processes previously describedherein, in which a plurality of first metal ions in the outer region ofthe glass article is exchanged with a plurality of second metal ions sothat the outer region comprises the plurality of the second metal ions.Each of the first metal ions has a first ionic radius and each of thesecond alkali metal ions has a second ionic radius. The second ionicradius is greater than the first ionic radius, and the presence of thelarger second alkali metal ions in the outer region creates thecompressive stress in the outer region.

At least one of the first metal ions and second metal ions are ions ofan alkali metal. The first ions may be ions of lithium, sodium,potassium, and rubidium. The second metal ions may be ions of one ofsodium, potassium, rubidium, and cesium, with the proviso that thesecond alkali metal ion has an ionic radius greater than the ionicradius than the first alkali metal ion.

In some embodiments, the glass is strengthened in a single ion exchangestep to produce the compressive stress profile shown in FIG. 2.Typically, the glass is immersed in a molten salt bath containing a saltof the larger alkali metal cation. In some embodiments, the molten saltbath contains or consists essentially of salts of the larger alkalimetal cation. However, small amounts—in some embodiments, less thatabout 10 wt %, in some embodiments, less than about 5 wt %, and, inother embodiments less than about 2 wt %—of salts of the smaller alkalimetal cation may be present in the bath. In other embodiments, salts ofthe smaller alkali metal cation may comprise at least about 30 wt %, orat least about 40 wt %, or from about 40 wt % up to about 75 wt % of theion exchange bath. This single ion exchange process may take place at atemperature of at least about 400° C. and, in some embodiments, at leastabout 440° C. and less than about 500° C., for a time sufficient toachieve the desired depth of compression DOC. In some embodiments, thesingle step ion exchange process may be conducted for at least eighthours, depending on the composition of the bath.

In another embodiment, the glass is strengthened in a two-step or dualion exchange method to produce the compressive stress profile shown inFIG. 3. The first step of the process, the glass is ion exchanged in thefirst molten salt bath described above. After completion of the firstion exchange, the glass is immersed in a second ion exchange bath. Thesecond ion exchange bath is different—i.e., separate from and, in someembodiments, having a different composition—from the first bath. In someembodiments, the second ion exchange bath contains only salts of thelarger alkali metal cation, although, in some embodiments small amountsof the smaller alkali metal cation (e.g., ≦2 wt %; ≦3 wt %) may bepresent in the bath. In addition, the immersion time and temperature ofthe second ion exchange step may differ from those of the first ionexchange step. In some embodiments, the second ion exchange step iscarried out at a temperature of at least about 350° C. and, in otherembodiments, at least about 380° C. up to about 450° C. The duration ofthe second ion exchange step is sufficient to achieve the desired depthd_(b) of the shallow segment b, in some embodiments, may be 30 minutesor less. In other embodiments, the duration of the second ion exchangestep is 15 minutes or less and, in some embodiments, in a range fromabout 10 minutes to about 60 minutes.

The second ion exchange bath is different than the first ion exchangebath, because the second ion exchange step is directed to delivering adifferent concentration of the larger cation or, in some embodiments, adifferent cation altogether, to the alkali aluminosilicate glass articlethan that provided in the first ion exchange step. In one or moreembodiments, the second ion exchange bath may comprise at least about95% by weight of a potassium composition that delivers potassium ions tothe alkali aluminosilicate glass article. In a specific embodiment, thesecond ion exchange bath may comprise from about 98% to about 99.5% byweight of the potassium composition. While it is possible that thesecond ion exchange bath only comprises at least one potassium salt, thesecond ion exchange bath may, in further embodiments, comprise 0-5% byweight, or about 0.5-2.5% by weight of at least one sodium salt, forexample, NaNO₃. In an exemplary embodiment, the potassium salt is KNO₃.In further embodiments, the temperature of the second ion exchange stepmay be 380° C. or greater and, in some embodiments, up to about 450° C.

The purpose of the second ion exchange step is to form a “spike”increase the compressive stress in the region immediately adjacent tothe surface of the glass article, as represented by portion b of thestress profile shown in FIG. 3.

The glass articles described herein may comprise or consist essentiallyof any glass that is chemically strengthened by ion exchange. In someembodiments, the glass is an alkali aluminosilicate glass.

In one embodiment, the alkali aluminosilicate glass comprises orconsists essentially of at least one of alumina and boron oxide, and atleast one of an alkali metal oxide and an alkali earth metal oxide,wherein −15 mol %≦(R₂O+R′O−Al₂O₃−ZrO₂)−B₂O₃≦4 mol %, where R is one ofLi, Na, K, Rb, and Cs, and R′ is at least one of Mg, Ca, Sr, and Ba. Insome embodiments, the alkali aluminosilicate glass comprises or consistsessentially of: from about 62 mol % to about 70 mol % SiO₂; from 0 mol %to about 18 mol % Al₂O₃; from 0 mol % to about 10 mol % B₂O₃; from 0 mol% to about 15 mol % Li₂O; from 0 mol % to about 20 mol % Na₂O; from 0mol % to about 18 mol % K₂O; from 0 mol % to about 17 mol % MgO; from 0mol % to about 18 mol % CaO; and from 0 mol % to about 5 mol % ZrO₂. Insome embodiments, the glass comprises alumina and boron oxide and atleast one alkali metal oxide, wherein −15 mol%≦(R₂O+R′O−Al₂O₃−ZrO₂)−B₂O₃ 4 mol %, where R is at least one of Li, Na,K, Rb, and Cs, and R′ is at least one of Mg, Ca, Sr, and Ba; wherein10≦Al₂O₃+B₂O₃+ZrO₂≦30 and 14≦R₂O+R′O≦25; wherein the silicate glasscomprises or consists essentially of: 62-70 mol. % SiO₂; 0-18 mol %Al₂O₃; 0-10 mol % B₂O₃; 0-15 mol % Li₂O; 6-14 mol % Na₂O; 0-18 mol %K₂O; 0-17 mol % MgO; 0-18 mol % CaO; and 0-5 mol % ZrO₂. The glass isdescribed in U.S. patent application Ser. No. 12/277,573 filed Nov. 25,2008, by Matthew J. Dejneka et al., and entitled “Glasses HavingImproved Toughness And Scratch Resistance,” and U.S. Pat. No. 8,652,978filed Aug. 17, 2012, by Matthew J. Dejneka et al., and entitled “GlassesHaving Improved Toughness And Scratch Resistance,” both claimingpriority to U.S. Provisional Patent Application No. 61/004,677, filed onNov. 29, 2008. The contents of all of the above patent and patentapplication are incorporated herein by reference in their entirety.

In another embodiment, the alkali aluminosilicate glass comprises orconsists essentially of: from about 60 mol % to about 70 mol % SiO₂;from about 6 mol % to about 14 mol % Al₂O₃; from 0 mol % to about 15 mol% B₂O₃; from 0 mol % to about 15 mol % Li₂O; from 0 mol % to about 20mol % Na₂O; from 0 mol % to about 10 mol % K₂O; from 0 mol % to about 8mol % MgO; from 0 mol % to about 10 mol % CaO; from 0 mol % to about 5mol % ZrO₂; from 0 mol % to about 1 mol % SnO₂; from 0 mol % to about 1mol % CeO₂; less than about 50 ppm As₂O₃; and less than about 50 ppmSb₂O₃; wherein 12 mol %≦Li₂O+Na₂O+K₂O 20 mol % and 0 mol %≦MgO+CaO≦10mol %. In some embodiments, the alkali aluminosilicate glass comprisesor consists essentially of: 60-70 mol % SiO₂; 6-14 mol % Al₂O₃; 0-3 mol% B₂O₃; 0-1 mol % Li₂O; 8-18 mol % Na₂O; 0-5 mol % K₂O; 0-2.5 mol % CaO;greater than 0 mol % to 3 mol % ZrO₂; 0-1 mol % SnO₂; and 0-1 mol %CeO₂, wherein 12 mol %<Li₂O+Na₂O+K₂O≦20 mol %, and wherein the silicateglass comprises less than 50 ppm As₂O₃. In some embodiments, the alkalialuminosilicate glass comprises or consists essentially of: 60-72 mol %SiO₂; 6-14 mol % Al₂O₃; 0-3 mol % B₂O₃; 0-1 mol % Li₂O; 0-20 mol % Na₂O;0-10 mol % K₂O; 0-2.5 mol % CaO; 0-5 mol % ZrO₂; 0-1 mol % SnO₂; and 0-1mol % CeO₂, wherein 12 mol %≦Li₂O+Na₂O+K₂O≦20 mol %, and wherein thesilicate glass comprises less than 50 ppm As₂O₃ and less than 50 ppmSb₂O₃. The glass is described in U.S. Pat. No. 8,158,543 by Sinue Gomezet al., entitled “Fining Agents for Silicate Glasses,” filed on Feb. 25,2009; U.S. Pat. No. 8,431,502 by Sinue Gomez et al., entitled “SilicateGlasses Having Low Seed Concentration,” filed Jun. 13, 2012; and U.S.Pat. No. 8,623,776, by Sinue Gomez et al., entitled “Silicate GlassesHaving Low Seed Concentration,” filed Jun. 19, 2013, all of which claimpriority to U.S. Provisional Patent Application No. 61/067,130, filed onFeb. 26, 2008. The contents of all of the above U.S. patents areincorporated herein by reference in their entirety.

In another embodiment, the alkali aluminosilicate glass comprises SiO₂and Na₂O, wherein the glass has a temperature T_(35kp) at which theglass has a viscosity of 35 kilo poise (kpoise), wherein the temperatureT_(breakdown) at which zircon breaks down to form ZrO₂ and SiO₂ isgreater than T_(35kp). In some embodiments, the alkali aluminosilicateglass comprises or consists essentially of: from about 61 mol % to about75 mol % SiO₂; from about 7 mol % to about 15 mol % Al₂O₃; from 0 mol %to about 12 mol % B₂O₃; from about 9 mol % to about 21 mol % Na₂O; from0 mol % to about 4 mol % K₂O; from 0 mol % to about 7 mol % MgO; andfrom 0 mol % to about 3 mol % CaO. The glass is described in U.S. Pat.No. 8,802,581 by Matthew J. Dejneka et al., entitled “Zircon CompatibleGlasses for Down Draw,” filed Aug. 10, 2010, and claiming priority toU.S. Provisional Patent Application No. 61/235,762, filed on Aug. 29,2009. The contents of the above patent and patent application areincorporated herein by reference in their entirety.

In another embodiment, the alkali aluminosilicate glass comprises atleast 50 mol % SiO₂ and at least one modifier selected from the groupconsisting of alkali metal oxides and alkaline earth metal oxides,wherein [(Al₂O₃ (mol %)+B₂O₃(mol %))/(Σalkali metal modifiers (mol%))]>1. In some embodiments, the alkali aluminosilicate glass comprisesor consists essentially of: from 50 mol % to about 72 mol % SiO₂; fromabout 9 mol % to about 17 mol % Al₂O₃; from about 2 mol % to about 12mol % B₂O₃; from about 8 mol % to about 16 mol % Na₂O; and from 0 mol %to about 4 mol % K₂O. In some embodiments, the glass comprises orconsists essentially of: at least 58 mol % SiO₂; at least 8 mol % Na₂O;from 5.5 mol % to 12 mol % B₂O₃; and Al₂O₃, wherein [(Al₂O₃ (mol%)+B₂O₃(mol %))/(Σalkali metal modifiers (mol %))]>1, Al₂O₃(mol%)>B₂O₃(mol %), 0.9<R₂O/Al₂O₃<1.3. The glass is described in U.S. Pat.No. 8,586,492, entitled “Crack And Scratch Resistant Glass andEnclosures Made Therefrom,” filed Aug. 18, 2010, by Kristen L. Barefootet al., and U.S. patent application Ser. No. 14/082,847, entitled “CrackAnd Scratch Resistant Glass and Enclosures Made Therefrom,” filed Nov.18, 2013, by Kristen L. Barefoot et al., both claiming priority to U.S.Provisional Patent Application No. 61/235,767, filed on Aug. 21, 2009.The contents of the above patent and patent applications areincorporated herein by reference in their entirety.

In another embodiment, the alkali aluminosilicate glass comprises SiO₂,Al₂O₃, P₂O₅, and at least one alkali metal oxide (R₂O), wherein0.75<[(P₂O₅ (mol %)+R₂O(mol %))/M₂O₃ (mol %)]<1.2, whereM₂O₃=Al₂O₃+B₂O₃. In some embodiments, the alkali aluminosilicate glasscomprises or consists essentially of: from about 40 mol % to about 70mol % SiO₂; from 0 mol % to about 28 mol % B₂O₃; from 0 mol % to about28 mol % Al₂O₃; from about 1 mol % to about 14 mol % P₂O₅; and fromabout 12 mol % to about 16 mol % R₂O and, in certain embodiments, fromabout 40 to about 64 mol % SiO₂; from 0 mol % to about 8 mol % B₂O₃;from about 16 mol % to about 28 mol % Al₂O₃; from about 2 mol % to about12 mol % P₂O₅; and from about 12 mol % to about 16 mol % R₂O. The glassis described in U.S. patent application Ser. No. 13/305,271 by Dana C.Bookbinder et al., entitled “Ion Exchangeable Glass with DeepCompressive Layer and High Damage Threshold,” filed Nov. 28, 2011, andclaiming priority to U.S. Provisional Patent Application No. 61/417,941,filed Nov. 30, 2010. The contents of the above patent applications areincorporated herein by reference in their entirety.

In still another embodiment, the alkali aluminosilicate glass comprisesat least about 50 mol % SiO₂ and at least about 11 mol % Na₂O, and has asurface compressive stress of at least about 900 MPa. In someembodiments, the glass further comprises Al₂O₃ and at least one of B₂O₃,K₂O, MgO and ZnO, wherein−340+27.1·Al₂O₃−28.7·B₂O₃+15.6·Na₂O−61.4·K₂O+8.1·(MgO+ZnO)≧0 mol %. Inparticular embodiments, the glass comprises or consists essentially of:from about 7 mol % to about 26 mol % Al₂O₃; from 0 mol % to about 9 mol% B₂O₃; from about 11 mol % to about 25 mol % Na₂O; from 0 mol % toabout 2.5 mol % K₂O; from 0 mol % to about 8.5 mol % MgO; and from 0 mol% to about 1.5 mol % CaO. The glass is described in U.S. patentapplication Ser. No. 13/533,298, by Matthew J. Dejneka et al., entitled“Ion Exchangeable Glass with High Compressive Stress,” filed Jun. 26,2012, and claiming priority to U.S. Provisional Patent Application No.61/503,734, filed Jul. 1, 2011. The contents of the above patentapplications are incorporated herein by reference in their entirety.

In other embodiments, the alkali aluminosilicate glass is ionexchangeable and comprises: at least about 50 mol % SiO₂; at least about10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃; and B₂O₃, whereinB₂O₃−(R₂O−Al₂O₃)≧3 mol %. In some embodiments, the glass comprises: atleast about 50 mol % SiO₂; at least about 10 mol % R₂O, wherein R₂Ocomprises Na₂O; Al₂O₃, wherein Al₂O₃(mol %)<R₂O(mol %); and from 3 mol 5to 4.5 mol % B₂O₃, wherein B₂O₃(mol %)−(R₂O(mol %)−Al₂O₃(mol %))≧3 mol%. In certain embodiments, the glass comprises or consists essentiallyof: at least about 50 mol % SiO₂; from about 9 mol % to about 22 mol %Al₂O₃; from about 3 mol % to about 10 mol % B₂O₃; from about 9 mol % toabout 20 mol % Na₂O; from 0 mol % to about 5 mol % K₂O; at least about0.1 mol % MgO, ZnO, or combinations thereof, wherein 0≦MgO≦6 and 0≦ZnO≦6mol %; and, optionally, at least one of CaO, BaO, and SrO, wherein 0 mol%≦CaO+SrO+BaO≦2 mol %. When ion exchanged, the glass, in someembodiments, has a Vickers crack initiation threshold of at least about10 kgf. Such glasses are described in U.S. patent application Ser. No.14/197,658, filed May 28, 2013, by Matthew J. Dejneka et al., entitled“Zircon Compatible, Ion Exchangeable Glass with High Damage Resistance,”which is a continuation of U.S. patent application Ser. No. 13/903,433,filed May 28, 2013, by Matthew J. Dejneka et al., entitled “ZirconCompatible, Ion Exchangeable Glass with High Damage Resistance,” bothclaiming priority to Provisional Patent Application No. 61/653,489,filed May 31, 2012. The contents of these patent applications areincorporated herein by reference in their entirety.

In some embodiments, the glass comprises: at least about 50 mol % SiO₂;at least about 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃, wherein−0.5 mol %≦Al₂O₃(mol %)−R₂O(mol %)≧2 mol %; and B₂O₃, and whereinB₂O₃(mol %)−(R₂O(mol %)−Al₂O₃(mol %))≧4.5 mol %. In other embodiments,the glass has a zircon breakdown temperature that is equal to thetemperature at which the glass has a viscosity of greater than about 40kPoise and comprises: at least about 50 mol % SiO₂; at least about 10mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃; and B₂O₃, wherein B₂O₃(mol%)−(R₂O(mol %)−Al₂O₃(mol %))≧4.5 mol %. In still other embodiments, theglass is ion exchanged, has a Vickers crack initiation threshold of atleast about 30 kgf, and comprises: at least about 50 mol % SiO₂; atleast about 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃, wherein−0.5 mol %≦Al₂O₃(mol %)−R₂O(mol %)≦2 mol %; and B₂O₃, wherein B₂O₃(mol%)−(R₂O(mol %)−Al₂O₃(mol %))≧4.5 mol %. Such glasses are described inU.S. patent application Ser. No. 13/903,398, by Matthew J. Dejneka etal., entitled “Ion Exchangeable Glass with High Damage Resistance,”filed May 28, 2013, claiming priority from U.S. Provisional PatentApplication No. 61/653,485, filed May 31, 2012. The contents of thesepatent applications are incorporated herein by reference in theirentirety.

In certain embodiments, the alkali aluminosilicate glass comprises atleast about 4 mol % P₂O₅, wherein (M₂O₃(mol %)/R_(x)O(mol %))<1, whereinM₂O₃=Al₂O₃+B₂O₃, and wherein R_(x)O is the sum of monovalent anddivalent cation oxides present in the alkali aluminosilicate glass. Insome embodiments, the monovalent and divalent cation oxides are selectedfrom the group consisting of Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O, MgO, CaO, SrO,BaO, and ZnO. In some embodiments, the glass is lithium-free andconsists essentially of from about 40 mol % to about 70 mol % SiO₂; fromabout 11 mol % to about 25 mol % Al₂O₃; from about 4 mol % to about 15mol % P₂O₅; from about 13 mol % to about 25 mol % Na₂O; from about 13 toabout 30 mol % R_(x)O, where wherein R_(x)O is the sum of the alkalimetal oxides, alkaline earth metal oxides, and transition metalmonoxides present in the glass; from about 11 to about 30 mol % M₂O₃,where M₂O₃=Al₂O₃+B₂O₃; from 0 mol % to about 1 mol % K₂O; from 0 mol %to about 4 mol % B₂O₃, and 3 mol % or less of one or more of TiO₂, MnO,Nb₂O₅, MoO₃, Ta₂O₅, WO₃, ZrO₂, Y₂O₃, La₂O₃, HfO₂, CdO, SnO₂, Fe₂O₃,CeO₂, As₂O₃, Sb₂O₃, Cl, and Br; the glass is lithium-free; and1.3<[(P₂O₅+R₂O)/M₂O₃]≦2.3, where R₂O is the sum of monovalent cationoxides present in the glass. The glass is described in U.S. patentapplication Ser. No. 13/678,013 by Timothy M. Gross, entitled “IonExchangeable Glass with High Crack Initiation Threshold,” filed Nov. 15,2012, and U.S. Pat. No. 8,756,262 by Timothy M. Gross, entitled “IonExchangeable Glass with High Crack Initiation Threshold,” filed Nov. 15,2012, both claiming priority to U.S. Provisional Patent Application No.61/560,434 filed Nov. 16, 2011. The contents of the above patent andapplications are incorporated herein by reference in their entirety.

In other embodiments, the alkali aluminosilicate glass comprises: fromabout 50 mol % to about 72 mol % SiO₂; from about 12 mol % to about 22mol % Al₂O₃; up to about 15 mol % B₂O₃; up to about 1 mol % P₂O₅; fromabout 11 mol % to about 21 mol % Na₂O; up to about 5 mol % K₂O; up toabout 4 mol % MgO; up to about 5 mol % ZnO; and up to about 2 mol % CaO.In some embodiments, the glass comprises: from about 55 mol % to about62 mol % SiO₂; from about 16 mol % to about 20 mol % Al₂O₃; from about 4mol % to about 10 mol % B₂O₃; from about 14 mol % to about 18 mol %Na₂O; from about 0.2 mol % to about 4 mol % K₂O; up to about 0.5 mol %MgO; up to about 0.5 mol % ZnO; and up to about 0.5 mol % CaO, whereinthe glass is substantially free of P₂O₅. In some embodiments,Na₂O+K₂O−Al₂O₃ 2.0 mol % and, in certain embodiments Na₂O+K₂O−Al₂O₃ 0.5mol %. In some embodiments, B₂O₃−(Na₂O+K₂O−Al₂O₃)>4 mol % and, incertain embodiments, B₂O₃−(Na₂O+K₂O−Al₂O₃)>1 mol %. In some embodiments,24 mol %≦RAlO₄≦45 mol %, and, in other embodiments, 28 mol %≦RAlO₄≦45mol %, where R is at least one of Na, K, and Ag. The glass is describedin U.S. Provisional Patent Application No. 61/909,049 by Matthew J.Dejneka et al., entitled “Fast Ion Exchangeable Glasses with HighIndentation Threshold,” filed Nov. 26, 2013, the contents of which areincorporated herein by reference in their entirety.

In some embodiments, the glasses described herein are substantially freeof at least one of arsenic, antimony, barium, strontium, bismuth, andtheir compounds. In other embodiments, the glasses may include up toabout 0.5 mol % Li₂O, or up to about 5 mol % Li₂O or, in someembodiments, up to about 10 mol % Li₂O. in still other embodiments, theglass may be free of Li₂O.

In some embodiments, the glasses described herein, when ion exchanged,are resistant to introduction of flaws by sharp or sudden impact.Accordingly, these ion exchanged glasses exhibit Vickers crackinitiation threshold of at least about 10 kilogram force (kgf). Incertain embodiments, these glasses exhibit a Vickers crack initiationthreshold of at least 20 kgf and, in some embodiments, at least about 30kgf, and up to 50 kgf.

The glasses described herein may, in some embodiments, be down-drawableby processes known in the art, such as slot-drawing, fusion drawing,re-drawing, and the like, and have a liquidus viscosity of at least 130kilopoise. In addition to those compositions listed hereinabove, variousother ion exchangeable alkali aluminosilicate glass compositions may beused.

The strengthened glasses described herein are considered suitable forvarious two- and three-dimensional shapes and may be utilized in variousapplications, and various thicknesses are contemplated herein. In someembodiments, the glass article has a thickness in a range from about 0.1mm up to about 2.0 mm. In some embodiments, the glass article has athickness in a range from about 0.1 mm up to about 1.0 mm and, incertain embodiments, from about 0.1 mm up to about 0.5 mm.

Strengthened glass articles may also be defined by their central tensionCT. In one or more embodiments, the strengthened glass articlesdescribed herein have a CT≦150 MPa, or a CT≦125 MPa, or CT≦100 MPa. Thecentral tension of the strengthened glass correlates to the frangiblebehavior of the strengthened glass article.

In another aspect, a method of making a strengthened glass articlehaving at least one compressive stress layer extending from a surface ofthe strengthened glass article to a depth of compression DOC of at leastabout 45 μm is provided. The method includes a first ion exchange stepin which an alkali aluminosilicate glass article is immersed in a firstion exchange bath at a temperature of greater than 400° C. for a timesufficient such that the compressive stress layer has a depth ofcompression of at least about 45 μm after the first ion exchange step.In some embodiments, it is preferable that the depth of compressionachieved after the first step be at least 50 μm. Even more preferableare compression depths DOC greater than 55 μm, or even 60 μm,particularly if the thickness of the glass exceeds 0.5 mm. In someembodiments, the depth of compression DOC of an individual compressivelayer (e.g., 120 in FIG. 1) is at least about 0.1·t, and in otherembodiments, at least about 0.15·t, and may be as great as 0.20·t.

Actual immersion times in the first ion exchange bath may depend uponfactors such as the temperature and/or composition of the ion exchangebath, the diffusivity of the cations within the glass, and the like.Accordingly, various time periods for ion exchange are contemplated asbeing suitable. In those instances in which potassium cations from theion exchange bath are exchanged for sodium cations in the glass, thebath typically comprises potassium nitrate (KNO₃). Here, the first ionexchange step, in some embodiments, may be conducted for a time of atleast 5 hours. Longer ion exchange periods for the first ion exchangestep may correlate with larger sodium ion contents in the first ionexchange bath. The desired sodium ion content in first ion exchange bathmay be achieved, for example, by including at least about 30% by weightor, in some embodiments, at least about 40% by weight of a sodiumcompound such as sodium nitrate (NaNO₃) or the like in the first ionexchange bath. In some embodiments, the sodium compound accounts forabout 40% to about 60% by weight of the first ion exchange bath. In anexemplary embodiment, the first ion exchange step is carried out at atemperature of about 440° C. or greater.

After the first ion exchange step is performed, the strengthened glassarticle may have a maximum compressive stress (CS) of at least 150 MPa.In further embodiments, the strengthened glass article may have a CS ofat least 200 MPa after the first ion exchange step, or a CS range ofabout 200 to about 400 MPa after the first ion exchange step. While thefirst ion exchange step minimally achieves a compressive layerdepth/depth of compression DOC of at least 45 μm, it is contemplatedthat the compressive stress layer may have a depth of 50 μm to 100 μmand, in some embodiments, 60 μm to 100 μm after the first ion exchangestep.

Following the first ion exchange step, a second ion exchange step may beconducted by immersing the alkali aluminosilicate glass article in asecond ion exchange bath at a temperature of at least 350° C. and up toabout 450° C. for a time sufficient to produce the shallow steep segmentwith a depth d_(b) of at least about 3 μm and up to about 15 μm. In someembodiments, the second ion exchange bath differs in composition and/ortemperature from the first ion exchange bath.

The second ion exchange step is a relatively rapid ion exchange stepthat yields a “spike” of compressive stress near the surface of theglass as depicted in FIG. 3. In one or more embodiments, the second ionexchange step may be conducted for a time of up to about 30 minutes or,in other embodiments, up to about 15 minutes or, in some embodiments, ina range from about 10 minutes to about 60 minutes.

The second ion exchange step is directed to delivering a different ionto the alkali aluminosilicate glass article than the ion provided by thefirst ion exchange step. The composition of the second ion exchange baththerefore differs from the first ion exchange bath. In some embodiments,the second ion exchange bath comprises at least about 95% by weight of apotassium composition (e.g., KNO₃) that delivers potassium ions to thealkali aluminosilicate glass article. In a specific embodiment, thesecond ion exchange bath may comprise from about 98% to about 99.5% byweight of the potassium composition. While it is possible that thesecond ion exchange bath only contains a potassium salt, the second ionexchange bath may, in further embodiments, comprise up to about 2% byweight or from about 0.5% to about 1.5% by weight of a sodium salt suchas, for example, NaNO₃. In further embodiments, the temperature of thesecond ion exchange step may be 390° C. or greater and up to about 450°C.

In some embodiments, the second ion exchange step may conclude thechemical strengthening procedure. The strengthened glass article mayhave a compressive stress (CS) of at least about 700 MPa following thesecond ion exchange step. In a further embodiment, the strengthenedglass article has a maximum compressive stress of about 500 to about1200 MPa, or about 700 to 1000 MPa after the second ion exchange step.While the second ion exchange step minimally achieves a compressivelayer DOC of at least about 70 μm, it is contemplated that thecompressive stress layer may have a DOC in a range from about 90 μm toabout 130 μm after the second ion exchange step.

Frangible behavior is characterized by at least one of: breaking of thestrengthened glass article (e.g., a plate or sheet) into multiple smallpieces (e.g., ≦1 mm); the number of fragments formed per unit area ofthe glass article; multiple crack branching from an initial crack in theglass article; violent ejection of at least one fragment to a specifieddistance (e.g., about 5 cm, or about 2 inches) from its originallocation; and combinations of any of the foregoing breaking (size anddensity), cracking, and ejecting behaviors. As used herein, the terms“frangible behavior” and “frangibility” refer to those modes of violentor energetic fragmentation of a strengthened glass article absent anyexternal restraints, such as coatings, adhesive layers, or the like.While coatings, adhesive layers, and the like may be used in conjunctionwith the strengthened glass articles described herein, such externalrestraints are not used in determining the frangibility or frangiblebehavior of the glass articles.

Examples of frangible behavior and non-frangible behavior ofstrengthened glass articles upon point impact with a scribe having asharp tungsten carbide (WC) tip are shown in FIGS. 13a and 13b . Thepoint impact test that is used to determine frangible behavior includesan apparatus that is delivered to the surface of the glass article witha force that is just sufficient to release the internally stored energypresent within the strengthened glass article. That is, the point impactforce is sufficient to create at least one new crack at the surface ofthe strengthened glass sheet and extend the crack through thecompressive stress CS region (i.e., depth of layer) into the region thatis under central tension CT. The impact energy needed to create oractivate the crack in a strengthened glass sheet depends upon thecompressive stress CS and depth of layer DOL of the article, and thusupon the conditions under which the sheet was strengthened (i.e., theconditions used to strengthen a glass by ion exchange). Otherwise, eachion exchanged glass plate shown in FIGS. 13a and 13b was subjected to asharp dart indenter (e.g., a scribe with a sharp WC point) contactsufficient to propagate a crack into the inner region of the plate, theinner region being under tensile stress. The force applied to the glassplate was just sufficient to reach the beginning of the inner region,thus allowing the energy that drives the crack to come from the tensilestresses in the inner region rather than from the force of the dartimpact on the outer surface. The degree of ejection may be determined,for example, by centering the glass sample on a grid, impacting thesample and measuring the ejection distance of individual pieces usingthe grid.

Referring to FIG. 14a , glass plate a can be classified as beingfrangible. In particular, glass plate a fragmented into multiple smallpieces that were ejected, and exhibited a large degree of crackbranching from the initial crack to produce the small pieces.Approximately 50% of the fragments are less than 1 mm in size, and it isestimated that about 8 to 10 cracks branched from the initial crack.Glass pieces were also ejected about 5 cm from original glass plate a,as seen in FIG. 14a . A glass article that exhibits any of the threecriteria (i.e., multiple crack branching, ejection, and extremefragmentation) described hereinabove is classified as being frangible.For example, if a glass exhibits excessive branching alone but does notexhibit ejection or extreme fragmentation as described above, the glassis still characterized as frangible.

Glass plates b, c, (FIG. 14b ) and d (FIG. 14a ) are classified as notfrangible. In each of these samples, the glass sheet has broken into asmall number of large pieces. Glass plate b (FIG. 14), for example, hasbroken into two large pieces with no crack branching; glass plate c(FIG. 14b ) has broken into four pieces with two cracks branching fromthe initial crack; and glass plate d (FIG. 14a ) has broken into fourpieces with two cracks branching from the initial crack. Based on theabsence of ejected fragments (i.e., no glass pieces forcefully ejectedmore than 2 inches from their original location), no visible fragmentsthat are less than or equal to 1 mm in size, and the minimal amount ofobserved crack branching, samples b, c, and d are classified asnon-frangible or substantially non-frangible.

Based on the foregoing, a frangibility index (Table 1) can beconstructed to quantify the degree of frangible or non-frangiblebehavior of a glass, glass ceramic, and/or a ceramic article upon impactwith another object. Index numbers, ranging from 1 for non-frangiblebehavior to 5 for highly frangible behavior, have been assigned todescribe different levels of frangibility or non-frangibility. Using theindex, frangibility can be characterized in terms of numerousparameters: 1) the percentage of the population of fragments having adiameter (i.e., maximum dimension) of less than 1 mm (“Fragment size” inTable 1); 2) the number of fragments formed per unit area (in thisinstance, cm²) of the sample (“Fragment density” in Table 1); 3) thenumber of cracks branching from the initial crack formed upon impact(“Crack branching” in Table 1); and 4) the percentage of the populationof fragments that is ejected upon impact more than about 5 cm (or about2 inches) from their original position (“Ejection” in Table 1).

TABLE 1 Criteria for determining the degree of frangibility andfrangibility index. Fragment Degree Frangi- Fragment density of fran-bility size (fragments/ Crack Ejection gibility index (% ≦ 1 mm) cm²)branching (% ≧ 5 cm) High 5 >20 >7 >9 >6 Medium 4 10 < n ≦ 20 5 < n ≦ 77 < n ≦ 9 4 < n ≦ 6 Low 3  5 < n ≦ 10 3 < n ≦ 5 5 < n ≦ 7 2 < n ≦ 4 None2 0 < n ≦ 5 1 < n ≦ 3 2 < n ≦ 5 0 < n ≦ 2 1  0 n ≦ 1 n ≦ 2  0

A frangibility index is assigned to a glass article if the article meetsat least one of the criteria associated with a particular index value.Alternatively, if a glass article meets criteria between two particularlevels of frangibility, the article may be assigned a frangibility indexrange (e.g., a frangibility index of 2-3). The glass article may beassigned the highest value of frangibility index, as determined from theindividual criteria listed in Table 1. In many instances, it is notpossible to ascertain the values of each of the criteria, such as thefragmentation density or percentage of fragments ejected more than 5 cmfrom their original position, listed in Table 1. The different criteriaare thus considered individual, alternative measures of frangiblebehavior and the frangibility index such that a glass article fallingwithin one criteria level will be assigned the corresponding degree offrangibility and frangibility index. If the frangibility index based onany of the four criteria listed in Table 1 is 3 or greater, the glassarticle is classified as frangible.

Applying the foregoing frangibility index to the samples shown in FIGS.13a and 13b , glass plate a fragmented into multiple ejected smallpieces and exhibited a large degree of crack branching from the initialcrack to produce the small pieces. Approximately 50% of the fragmentsare less than 1 mm in size and it is estimated that about 8 to 10 cracksbranched from the initial crack. Based upon the criteria listed in Table1, glass plate a has a frangibility index of between about 4-5, and isclassified as having a medium-high degree of frangibility.

A glass article having a frangibility index of less than 3 (lowfrangibility) may be considered to be non-frangible or substantiallynon-frangible. Glass plates b, c, and d each lack fragments having adiameter of less than 1 mm, multiple branching from the initial crackformed upon impact and fragments ejected more than 5 cm from theiroriginal position. Glass plates b, c, and d are non-frangible and thushave a frangibility index of 1 (not frangible).

As previously discussed, the observed differences in behavior betweenglass plate a, which exhibited frangible behavior, and glass plates b,c, and d, which exhibited non-frangible behavior, in FIGS. 13a and 13bcan be attributed to differences in central tension CT among the samplestested. The possibility of such frangible behavior is one considerationin designing various glass products, such as cover plates or windows forportable or mobile electronic devices such as cellular phones,entertainment devices, and the like, as well as for displays forinformation terminal (IT) devices, such as laptop computers. Moreover,the depth of the compression layer DOL and the maximum value ofcompressive stress CS that can be designed into or provided to a glassarticle are limited by such frangible behavior.

Accordingly, the strengthened glass articles described herein, in someembodiments, exhibit a frangibility index of less than 3 when subjectedto a point impact sufficient to break the strengthened glass article. Inother embodiments, non-frangible strengthened glass articles may achievea frangibility index of less than 2 or less than 1.

The strengthened glass articles described herein demonstrate improvedfracture resistance when subjected to repeated drop tests. The purposeof such drop tests is to characterize the performance of such glassarticles in normal use as display windows or cover plates for handheldelectronic devices such as cell phones, smart phones, and the like.

A typical ball drop test concept that is currently in use is shown inFIG. 15a . The ball drop test assembly 250 includes a solid, hardsubstrate 212 such as a granite slab or the like and a steel ball 230 ofpredetermined mass and diameter. A glass sample 220 having an uppersurface 222 and lower surface 224 is secured to the substrate 212, and apiece of sandpaper 214 having the desired grit is placed on the uppersurface of the glass sample 220 opposite the substrate 212. Thesandpaper 214 is placed on the glass sample 220 such that the roughenedsurface 214 a of the sandpaper contacts the upper surface 222 of theglass sample 220. The steel ball 230 is allowed to fall freely from apredetermined height h onto the sandpaper 214. The upper surface 222 orcompression face of the glass sample 220 makes contact with theroughened surface 214 a of the sandpaper 214, introducing cracks intothe surface of the upper surface/compression face 222. The height h maybe increased incrementally until either a maximum height is reached orthe glass sample fractures.

The ball drop test 250 described hereinabove does not represent the truebehavior of glass when dropped onto and contacted by a rough surface.Instead, it is known that the face of the glass bends outward intension, rather than inward in compression as shown in FIG. 15 a.

An inverted ball on sandpaper (IBoS) test is a dynamic component leveltest that mimics the dominant mechanism for failure due to damageintroduction plus bending that typically occurs in strengthened glassarticles that are used in mobile or hand held electronic devices, asschematically shown in FIG. 15c . In the field, damage introduction (ain FIG. 15c ) occurs on the top surface of the glass. Fracture initiateson the top surface of the glass and damage either penetrates thecompressive layer (b in FIG. 15c ) or the fracture propagates frombending on the top surface or from center tension (c in FIG. 15c ). TheIBoS test is designed to simultaneously introduce damage to the surfaceof the glass and apply bending under dynamic load.

An IBoS test apparatus is schematically shown in FIG. 15b . Apparatus200 includes a test stand 210 and a ball 230. Ball 230 is a rigid orsolid ball such as, for example, a stainless steel ball, or the like. Inone embodiment, ball 230 is a 4.2 gram stainless steel ball havingdiameter of 10 mm. The ball 230 is dropped directly onto the glasssample 218 from a predetermined height h. Test stand 210 includes asolid base 212 comprising a hard, rigid material such as granite or thelike. A sheet 214 having an abrasive material disposed on a surface isplaced on the upper surface of the solid base 212 such that surface withthe abrasive material faces upward. In some embodiments, sheet 214 issandpaper having a 30 grit surface and, in other embodiments, a 180 gritsurface. Glass sample 218 is held in place above sheet 214 by sampleholder 215 such that an air gap 216 exists between glass sample 218 andsheet 214. The air gap 216 between sheet 214 and glass sample 218 allowsthe glass sample 218 to bend upon impact by ball 230 and onto theabrasive surface of sheet 214. In one embodiment, the glass sample 218is clamped across all corners to keep bending contained only to thepoint of ball impact and to ensure repeatability. In some embodiments,sample holder 214 and test stand 210 are adapted to accommodate samplethicknesses of up to about 2 mm. The air gap 216 is in a range fromabout 50 μm to about 100 μm. An adhesive tape 220 may be used to coverthe upper surface of the glass sample to collect fragments in the eventof fracture of the glass sample 218 upon impact of ball 230.

Various materials may be used as the abrasive surface. In a oneparticular embodiment, the abrasive surface is sandpaper, such assilicon carbide or alumina sandpaper, engineered sandpaper, or anyabrasive material known to those skilled in the art for havingcomparable hardness and/or sharpness. In some embodiments, sandpaperhaving 30 grit may be used, as it has a surface topography that is moreconsistent than either concrete or asphalt, and a particle size andsharpness that produces the desired level of specimen surface damage.

In one aspect, a method 300 of conducting the IBoS test using theapparatus 200 described hereinabove is shown in FIG. 15d . In Step 310,a glass sample (218 in FIG. 15b ) is placed in the test stand 210,described previously and secured in sample holder 215 such that an airgap 216 is formed between the glass sample 218 and sheet 214 with anabrasive surface. Method 300 presumes that the sheet 214 with anabrasive surface has already been placed in test stand 210. In someembodiments, however, the method may include placing sheet 214 in teststand 210 such that the surface with abrasive material faces upward. Insome embodiments (Step 310 a), an adhesive tape 220 is applied to theupper surface of the glass sample 218 prior to securing the glass sample218 in the sample holder 210.

In Step 320, a solid ball 230 of predetermined mass and size is droppedfrom a predetermined height h onto the upper surface of the glass sample218, such that the ball 230 impacts the upper surface (or adhesive tape220 affixed to the upper surface) at approximately the center (i.e.,within 1 mm, or within 3 mm, or within 5 mm, or within 10 mm of thecenter) of the upper surface. Following impact in Step 320, the extentof damage to the glass sample 218 is determined (Step 330). Aspreviously described hereinabove, herein, the term “fracture” means thata crack propagates across the entire thickness and/or entire surface ofa substrate when the substrate is dropped or impacted by an object.

In method 300, the sheet 218 with the abrasive surface may be replacedafter each drop to avoid “aging” effects that have been observed inrepeated use of other types (e.g., concrete or asphalt) of drop testsurfaces.

Various predetermined drop heights h and increments are typically usedin method 300. The test may, for example, utilize a minimum drop heightto start (e.g., about 10-20 cm). The height may then be increased forsuccessive drops by either a set increment or variable increments. Thetest described in method 300 is stopped once the glass sample 218 breaksor fractures (Step 331). Alternatively, if the drop height h reaches themaximum drop height (e.g., about 100 cm) without glass fracture, thedrop test of method 300 may also be stopped, or Step 320 may be repeatedat the maximum height until fracture occurs.

In some embodiments, IBoS test of method 300 is performed only once oneach glass sample 218 at each predetermined height h. In otherembodiments, however, each sample may be subjected to multiple tests ateach height.

If fracture of the glass sample 218 has occurred (Step 331 in FIG. 15d), the IBoS test according to method 300 is ended (Step 340). If nofracture resulting from the ball drop at the predetermined drop heightis observed (Step 332), the drop height is increased by a predeterminedincrement (Step 334)—such as, for example 5, 10, or 20 cm—and Steps 320and 330 are repeated until either sample fracture is observed (331) orthe maximum test height is reached (336) without sample fracture. Wheneither Step 331 or 336 is reached, the test according to method 300 isended.

When subjected to the inverted ball on sandpaper (IBoS) test describedabove, the strengthened glasses that have undergone the dual ionexchange process described hereinabove have at least about a 60%survival rate when the ball is dropped onto the surface of the glassfrom a height of 80 cm. For example, a strengthened glass article isdescribed as having a 60% survival rate when dropped from a given heightwhen three of five identical (or nearly identical) samples (i.e., havingapproximately the same composition and, when strengthened, approximatelythe same CS and DOC or DOL) survive the IBoS drop test without fracturewhen dropped from the prescribed height (here 80 cm). In otherembodiments, the survival rate in the 80 cm IBoS test of thestrengthened glasses that have undergone the dual ion exchange processis at least about 70%, in other embodiments, at least about 80%, and, instill other embodiments, at least about 90%. In other embodiments, thesurvival rate of the strengthened glasses dropped from a height of 100cm in the IBoS test is at least about 60%, in other embodiments, atleast about 70%, in still other embodiments, at least about 80%, and, inother embodiments, at least about 90%.

To determine the survivability rate of the strengthened glass articlewhen dropped from a predetermined height using the IBoS test method andapparatus described hereinabove, at least five identical (or nearlyidentical) samples (i.e., having approximately the same composition andapproximately the same CS and DOC or DOL) of the strengthened glass aretested, although larger numbers (e.g., 10, 20, 30, etc.) of samples maybe subjected to testing to raise the confidence level of the testresults. Each sample is dropped a single time from the predeterminedheight (e.g., 80 cm) or, alternatively, dropped from progressivelyhigher heights without fracture until the predetermined height isreached, and visually (i.e., with the naked eye) examined for evidenceof fracture (crack formation and propagation across the entire thicknessand/or entire surface of a sample). A sample is deemed to have“survived” the drop test if no fracture is observed after being droppedfrom the predetermined height, and a sample is deemed to have “failed(or “not survived”) if fracture is observed when the sample is droppedfrom a height that is less than or equal to the predetermined height.The survivability rate is determined to be the percentage of the samplepopulation that survived the drop test. For example, if 7 samples out ofa group of 10 did not fracture when dropped from the predeterminedheight, the survivability rate of the glass would be 70%.

The strengthened glass articles described herein also demonstrateimproved surface strength when subjected to abraded ring-on-ring (AROR)testing. The strength of a material is defined as the stress at whichfracture occurs. The abraded ring-on-ring test is a surface strengthmeasurement for testing flat glass specimens, and ASTM C1499-09(2013),entitled “Standard Test Method for Monotonic Equibiaxial FlexuralStrength of Advanced Ceramics at Ambient Temperature,” serves as thebasis for the ring-on-ring abraded ROR test methodology describedherein. The contents of ASTM C1499-09 are incorporated herein byreference in their entirety. In one embodiment, the glass specimen isabraded prior to ring-on-ring testing with 90 grit silicon carbide (SiC)particles that are delivered to the glass sample using the method andapparatus described in Annex A2, entitled “abrasion Procedures,” of ASTMC158-02(2012), entitled “Standard Test Methods for Strength of Glass byFlexure (Determination of Modulus of Rupture). The contents of ASTMC158-02 and the contents of Annex 2 in particular are incorporatedherein by reference in their entirety.

Prior to ring-on-ring testing a surface of the glass sample is abradedas described in ASTM C158-02, Annex 2, to normalize and/or control thesurface defect condition of the sample using the apparatus shown inFigure A2.1 of ASTM C158-02. The abrasive material is sandblasted ontothe sample surface at a load of 15 psi using an air pressure of 304 kPa(44 psi). After air flow is established, 5 cm³ of abrasive material isdumped into a funnel and the sample is sandblasted for 5 seconds afterintroduction of the abrasive material.

For the ring-on-ring test, a glass specimen having at least one abradedsurface 412 is placed between two concentric rings of differing size todetermine equibiaxial flexural strength (i.e., the maximum stress that amaterial is capable of sustaining when subjected to flexure between twoconcentric rings), as schematically shown in FIG. 19. In the abradedring-on-ring configuration 400, the abraded glass specimen 410 issupported by a support ring 420 having a diameter D₂. A force F isapplied by a load cell (not shown) to the surface of the glass specimenby a loading ring 430 having a diameter D₁.

The ratio of diameters of the loading ring and support ring D₁/D₂ may bein a range from about 0.2 to about 0.5. In some embodiments, D₁/D₂ isabout 0.5. Loading and support rings 430, 420 should be alignedconcentrically to within 0.5% of support ring diameter D₂. The load cellused for testing should be accurate to within ±1% at any load within aselected range. In some embodiments, testing is carried out at atemperature of 23±2° C. and a relative humidity of 40±10%.

For fixture design, the radius r of the protruding surface of theloading ring 430, h/2≦r≦3 h/2, where h is the thickness of specimen 410.Loading and support rings 430, 420 are typically made of hardened steelwith hardness HR_(c)>40. ROR fixtures are commercially available.

The intended failure mechanism for the ROR test is to observe fractureof the specimen 410 originating from the surface 430 a within theloading ring 430. Failures that occur outside of this region—i.e.,between the loading rings 430 and support rings 420—are omitted fromdata analysis. Due to the thinness and high strength of the glassspecimen 410, however, large deflections that exceed ½ of the specimenthickness h are sometimes observed. It is therefore not uncommon toobserve a high percentage of failures originating from underneath theloading ring 430. Stress cannot be accurately calculated withoutknowledge of stress development both inside and under the ring(collected via strain gauge analysis) and the origin of failure in eachspecimen. AROR testing therefore focuses on peak load at failure as themeasured response.

The strength of glass depends on the presence of surface flaws. However,the likelihood of a flaw of a given size being present cannot beprecisely predicted, as the strength of glass is statistical in nature.A Weibull probability distribution is therefore generally used as astatistical representation of the data obtained.

In some embodiments, the strengthened glass described herein has asurface or equibiaxial flexural strength of at least 10 kgf and up toabout 50 kgf as determined by abraded ring-on-ring testing. In otherembodiments, the surface strength is at least 20 kgf, and in still otherembodiments, at least 30 kgf. A plot of AROR data as a function ofsample thickness for two strengthened alkali aluminosilicate glasses isshown in FIG. 20. Strengthened glass A, which is described in U.S.patent application Ser. No. 13/305,271, and having a nominal compositionof about 57 mol % SiO₂, 0 mol % B₂O₃, about 17 mol % Al₂O₃, about 7%P₂O₅, about 17 mol % Na₂O, about 0.02 mol % K₂O, and about 3 mol % MgO,exhibits a compressive stress profile corresponding to that shown inFIG. 3 resulting from the two-step ion exchange process describedherein. Strengthened glass B, which is described in U.S. patentapplication Ser. No. 13/903,433, and having a nominal composition ofabout 68 mol % SiO₂, about 13 mol % Al₂O₃, about 4 mol % B₂O₃, about 14mol % Na₂O, about 0.01 mol % K₂O, and about 2 mol % MgO, does notexhibit a compressive stress profile corresponding to that shown in FIG.3. As can be seen in FIG. 20, the two step ion exchange process resultsin higher surface strength as determined by AROR measurements.

EXAMPLES

The following examples illustrate the features and advantages describedherein and are no way intended to limit the disclosure and appendedclaims hereto.

Compressive Stress Profiles

Using the methods described by Roussev I and Roussev II, referencedhereinabove, glass samples of various thicknesses were ion exchanged andtheir respective compressive stress profiles were determined. Spectra ofbound optical modes for TM and TE polarization are collected via prismcoupling techniques, and used in their entirety to obtain detailed andprecise TM and TE refractive index profiles n_(TM)(z) and n_(TE)(z).Detailed index profiles are obtained by fitting the measured modespectra to numerically calculated spectra of pre-defined functionalforms that describe the shapes of the index profiles and obtaining theparameters of the functional forms from the best fit. The glass sampleshad compositions described in U.S. patent application Ser. No.13/678,013 by Timothy M. Gross. Samples having thicknesses of 0.4 mm,0.5 mm, 0.7 mm, 0.8 mm, and 1.0 mm were studied. The results of theseion exchange studies are summarized in Table 2.

TABLE 2 Results of ion exchange studies. IOX₁ and IOX₂ refer to thefirst and second ion exchange steps, respectively. Sample a b c d eThickness (mm) 0.4 0.4 0.4 0.5 0.5 IOX₁ Time (hr) 9 10 11.25 5.8 8.3 T(°C.) 440 440 440 440 440 Wt % 52/48 52/48 52/48 37/63 52/48 NaNO₃/KNO₃DOC (μm) 63 65 67 57 66 CS (MPa) 232 232 227 255 243 Slope A 3.4(MPa/μm) IOX₂ Time (min) 12 T(° C.) 390 Wt %  1/99 NaNO₃/KNO₃ DOC (μm)61 CS (MPa) 846 Slope A 89 (MPa/μm) Slope B 3.75 (MPa/μm) Transition8-16 region (μm) Sample k f g h l Thickness (mm) 0.55 0.7 0.8 0.8 0.8IOX₁ Time (hr) 7.75 8.5 8.8 8.8 48 T(° C.) 450 450 440 440 450 Wt %40/60 45/55 37/63 37/63 69/31 NaNO₃/KNO₃ DOC (μm) 73 75 72 72 142 CS(MPa) 268 281 358 358 146 Slope A 3.7 3.75 5.1 5.1 1.03 (MPa/μm) IOX₂Time (min) 12 12 12 24 T(° C.) 390 390 390 390 Wt %  0.5/99.5  1/99 1/99  1/99 NaNO₃/KNO₃ DOC (μm) 70 72 70 70 CS (MPa) 896 842 861 877Slope A 86 85 78 52 (MPa/μm) Slope B 3.7 3.75 4.65 5 (MPa/μm) Transition8-16 7-15 7-12 8-15 region C (μm) Sample m i j Thickness (mm) 0.8 0.91.0 IOX₁ Time (hr) 65 7.5 11 T(° C.) 450 450 440 Wt % 69/31 38/62 37/63NaNO₃/KNO₃ DOC (μm) 153 82 CS (MPa) 140 359 Slope A 0.92 5.3 (MPa/μm)IOX₂ Time (min) 18 12 T(° C.) 390 390 Wt %  2/98  1/99 NaNO₃/KNO₃ DOC(μm) 73 80 CS (MPa) 746 860 Slope A 52 73 (MPa/μm) Slope B 4 5.3(MPa/μm) Transition 8-16 region C (μm)i) 0.4 mm Thickness

Sample a was ion exchanged at 440° C. for 9 hours in a molten salt bathcontaining 52% NaNO₃ and 48% KNO₃ by weight. Following ion exchange, theTE and TM mode spectra were measured and the compressive stress profilewas determined therefrom. FIG. 4a shows TE (1) and TM (2) index profilesdetermined from the mode spectra, and FIG. 4b shows the compressivestress profile. The compressive stress profile has a single linearportion analogous to that shown in FIG. 2. The compressive stress CS atthe surface and the depth of compression DOC of sample a were determinedto be 232 MPa and 63 μm, respectively.

Sample b was ion exchanged at 440° C. for 10 hours in a molten salt bathcontaining 52% NaNO₃ and 48% KNO₃ by weight. Following ion exchange, theTE and TM mode spectra were measured and the compressive stress profilewas determined therefrom. FIG. 5a shows TE (1) and TM (2) index profilesdetermined from the mode spectra, and FIG. 5b shows the compressivestress profile determined from the mode spectra. The compressive stressprofile has a single linear portion analogous to that shown in FIG. 2.The compressive stress CS at the surface and the depth of compression ofsample b were determined to be 232 MPa and 65 μm, respectively.

Sample b was then subjected to a second ion exchange at 390° C. for 12minutes in a molten salt bath containing 1% NaNO₃ and 99% KNO₃ byweight. FIG. 5c shows the compressive stress profile determined from themode spectra. The compressive stress profile has a first linear segmentA extending from the surface of the glass (0 μm depth) to the beginningof a transition region C at about 8 μm and a second linear segment Bextending from the end of the transition region C at about 16 μm. Thecompressive stress profile shown in FIG. 5c is analogous to the stressprofile schematically shown in FIG. 3. The compressive stress CS at thesurface of the sample and the depth of compression were determined to be846 MPa and 61 μm, respectively. The slope of segment B of the stressprofile is approximately 3.75 MPa/um, whereas the slope of segment A was89 MPa/um. The transition region C from segment A to segment B rangedfrom a depth of about 8 μm to about 16 μm.

Sample c was ion exchanged at 440° C. for 11.25 hours in a molten saltbath containing 52% NaNO₃ and 48% KNO₃ by weight. Following ionexchange, the TE and TM index profiles determined from the mode spectrawere measured and the compressive stress profile was determinedtherefrom. FIG. 6a shows TE (1) and TM (2) mode spectra, and FIG. 6bshows the compressive stress profile determined from the mode spectra.The compressive stress profile has a single linear portion analogous tothat shown in FIG. 2. The compressive stress CS at the surface and thedepth of compression of sample c were determined to be 227 MPa and 67μm, respectively.

ii) 0.5 mm Thickness

Sample d was ion exchanged at 440° C. for 5.8 hours in a molten saltbath containing 37% NaNO₃ and 63% KNO₃ by weight. Following ionexchange, the TE and TM index profiles determined from the mode spectrawere measured and the compressive stress profile was determinedtherefrom. FIG. 7a shows TE (1) and TM (2) mode spectra, and FIG. 7bshows the compressive stress profile determined from the mode spectra.The compressive stress profile has a single linear portion analogous tothat shown in FIG. 2. The compressive stress CS at the surface and thedepth of compression of sample d were determined to be 255 MPa and 57μm, respectively.

Sample e was ion exchanged at 440° C. for 8.3 hours in a molten saltbath containing 52% NaNO₃ and 48% KNO₃ by weight. Following ionexchange, the TE and TM index profiles determined from the mode spectrawere measured and the compressive stress profile was determinedtherefrom. FIG. 8a shows the TE (1) and TM (2) mode spectra, and FIG. 8bshows the compressive stress profile determined from the index profilesderived from the mode spectra. The compressive stress profile has asingle linear portion analogous to that shown in FIG. 2. The compressivestress CS at the surface and the depth of compression of sample e weredetermined to be 243 MPa and 66 μm, respectively.

iii) 0.55 mm Thickness

Sample k was first ion exchanged at 450° C. for 7.75 hours in a moltensalt bath containing approximately 40% NaNO₃ and 60% KNO₃ by weight.Following ion exchange, the TE and TM mode spectra were measured and thecompressive stress profile was determined therefrom. FIG. 13a shows thecompressive stress profile determined from the index profiles derivedfrom the mode spectra. The compressive stress profile has a singlelinear portion analogous to that shown in FIG. 2. The compressive stressCS at the surface and the depth of compression of sample k after thefirst ion exchange were determined to be 268 MPa and 73 μm,respectively. The slope of the linear compressive stress profile was 3.7MPa/um.

Sample k was then subjected to a second ion exchange at 390° C. for 12minutes in a molten salt bath containing about 0.5% NaNO₃ and 99.5% KNO₃by weight. FIG. 13b shows the compressive stress profile determined fromthe mode spectra. Following the second ion exchange, the compressivestress profile had a first linear segment A extending from the surfaceof the glass to a transition region C at about 8 μm and a second linearsegment B extending from the transition region C at about 16 μm to thedepth of compression DOC. The compressive stress profile in FIG. 13b isanalogous to the stress profile schematically shown in FIG. 3. Thecompressive stress CS at the surface and the depth of compression ofsample k after the second ion exchange were determined to be 896 MPa and70 μm, respectively. The slope of segment B remained at approximately3.7 MPa/um, whereas the slope of portion A increased to about 86 MPa/um.The transition region C occurred over a range from a depth of about 8 mto about 16 μm.

iv) 0.7 mm Thickness

Sample f was first ion exchanged at 450° C. for 8.5 hours in a moltensalt bath containing 45% NaNO₃ and 55% KNO₃ by weight. Following ionexchange, the TE and TM mode spectra were measured and the compressivestress profile was determined therefrom. FIG. 9a shows the compressivestress profile determined from the mode spectra. The compressive stressprofile has a single linear portion analogous to that shown in FIG. 2.The compressive stress CS at the surface and the depth of compression ofsample f after the first ion exchange were determined to be 281 MPa and75 μm, respectively. The slope of the linear compressive stress profilewas 3.75 MPa/um.

Sample f was then subjected to a second ion exchange at 390° C. for 12minutes in a molten salt bath containing 1% NaNO₃ and 99% KNO₃ byweight. FIG. 9b shows the compressive stress profile determined from theTE and TM mode spectra. Following the second ion exchange, thecompressive stress profile had a first linear segment A extending fromthe surface of the glass to a transition region C at about 7 μm and asecond linear segment B extending from the transition region C at about15 μm to the depth of compression DOC. The CS profile is analogous tothe stress profile schematically shown in FIG. 3. The compressive stressCS at the surface and the depth of compression of sample f after thesecond ion exchange were determined to be 842 MPa and 72 μm,respectively. The slope of segment A increased to 85 MPa/um whereas theslope of segment B remained at approximately 3.75 MPa/um. The transitionregion C ranged from a depth of about 7 μm to about 15 μm.

iv) 0.8 mm Thickness

Samples g and h were first ion exchanged at 440° C. for 8.8 hours in amolten salt bath containing 37% NaNO₃ and 63% KNO₃ by weight. Followingion exchange, the TE and TM mode spectra were measured and thecompressive stress profile was determined therefrom. FIG. 10a shows thecompressive stress profile of sample g determined from the mode spectrafollowing the first ion exchange. The compressive stress profile has asingle linear portion analogous to that shown in FIG. 2. The compressivestress CS at the surface and the depth of compression of sample g afterthe first ion exchange were determined to be 358 MPa and 72 μm,respectively. The slope of the linear compressive stress profile was 5.1MPa/um.

Sample g was then subjected to a second ion exchange at 390° C. for 12minutes in a molten salt bath containing 1% NaNO₃ and 99% KNO₃ byweight. FIG. 10b shows the compressive stress profile determined fromthe TE and TM mode spectra. Following the second ion exchange, thecompressive stress profile had a first linear segment or portion Aextending from the surface of the glass to a transition region and asecond linear segment B extending from a transition region C to thedepth of compression DOC. The compressive stress profile is analogous tothe stress profile schematically shown in FIG. 3. The compressive stressCS at the surface and the depth of compression of sample g after thesecond ion exchange were determined to be 861 MPa and 70 μm,respectively. The slope of segment B was 4.65 MPa/um, whereas the slopeof segment A was 78 MPa/um. The transition region C from segment A tosegment B occurred over a range of depths from about 7 μm to about 12μm.

Following the first ion exchange, sample h was subjected to a second ionexchange at 390° C. for 24 minutes in a molten salt bath containing 1%NaNO₃ and 99% KNO₃ by weight. Following the second ion exchange, thecompressive stress profile had a first linear segment A extending fromthe surface of the glass to a depth of about 5 μm and a second linearsegment B extending from the upper boundary of a transition region C ata depth of about 15 um to a depth of 70 μm. The two segment CS profileis analogous to the stress profile schematically shown in FIG. 3. Thecompressive stress CS at the surface and the depth of compression ofsample g after the second ion exchange were determined to be 877 MPa and70 μm, respectively. The slope of segment B was about 5 MPa/um, whereasthe slope of segment A was 52 MPa/um. The transition region C fromsegment A to segment B occurred over a range of depths from about 8 μmto about 15 μm.

Sample l was first ion exchanged at 450° C. for 48 hours in a moltensalt bath containing 69% NaNO₃ and 31% KNO₃ by weight. Following ionexchange, the TE and TM mode spectra were measured and the compressivestress profile was determined therefrom. FIG. 16 shows the compressivestress profile determined from the TE and TM mode spectra. Thecompressive stress profile has a single linear portion analogous to thatshown in FIG. 2. The compressive stress CS at the surface and the depthof compression of sample l after the first ion exchange were determinedto be 146 MPa and 142 μm, respectively. The slope of the linearcompressive stress profile was 1.03 MPa/um.

Sample m was first ion exchanged at 450° C. for 65 hours in a moltensalt bath containing 69% NaNO₃ and 31% KNO₃ by weight. Following ionexchange, the TE and TM mode spectra were measured and the compressivestress profile was determined therefrom. FIG. 17 shows the compressivestress profile determined from the mode spectra. The compressive stressprofile has a single linear portion analogous to that shown in FIG. 2.The compressive stress CS at the surface and the depth of compression ofsample m after the first ion exchange were determined to be 140 MPa and153 μm, respectively. The slope of the linear compressive stress profilewas 0.92 MPa/um.

0.9 mm Thickness

Sample i was first ion exchanged at approximately 450° C. for about 7.5hours in a molten salt bath containing 38% NaNO₃ and 62% KNO₃ by weight.Following ion exchange, the TE and TM mode spectra were measured and thecompressive stress profile was determined therefrom.

Sample i was then subjected to a second ion exchange at 390° C. for 18minutes in a molten salt bath containing 2% NaNO₃ and 98% KNO₃ byweight. FIG. 11 shows the compressive stress profile determined from theTE and TM mode spectra. Following the second ion exchange, thecompressive stress profile had a first linear segment A and a secondlinear segment B, which is analogous to the stress profile schematicallyshown in FIG. 3. The compressive stress CS at the surface and the depthof compression of sample h after the second ion exchange were determinedto be 746 MPa and 73 μm, respectively. The slope of segment A wasapproximately 52 MPa/um, whereas the slope of segment B was about 4MPa/um.

1.0 mm Thickness

Sample j was first ion exchanged at 440° C. for 11 hours in a moltensalt bath containing 37% NaNO₃ and 63% KNO₃ by weight. Following ionexchange, the TE and TM mode spectra were measured and the compressivestress profile was determined therefrom. FIG. 12a shows the compressivestress profile of sample j determined from the mode spectra followingthe first ion exchange. The compressive stress profile has a singlelinear segment analogous to the stress profile shown in FIG. 2. Thecompressive stress CS at the surface and the depth of compression ofsample j after the first ion exchange were determined to be 359 MPa and82 μm, respectively. The slope of the linear compressive stress profilewas 5.3 MPa/um.

Sample j was then subjected to a second ion exchange at 390° C. for 12minutes in a molten salt bath containing 1% NaNO₃ and 99% KNO₃ byweight. FIG. 12b shows the compressive stress profile determined fromthe TE and TM mode spectra. Following the second ion exchange, thecompressive stress profile had a first linear segment A extending fromthe surface of the glass to the beginning of a transition region C atabout 8 μm and a second linear segment B extending from the end oftransition region C at about 16 μm to the depth of compression DOC. Thisbehavior is analogous to the stress profile schematically shown in FIG.3. The compressive stress CS at the surface and the depth of compressionof sample j after the second ion exchange were determined to be 860 MPaand 80 μm, respectively. The slope of segment A increased to 73 MPa/umfollowing the second ion exchange, whereas the slope of segment Bremained at approximately 5.3 MPa/um. The transition region C from slopeA to slope B occurred over a range of depths from about 8 μm and about16 μm.

Inverted Ball on Sandpaper (IBoS) Test

Three different types of glasses were subjected to inverted ball drop onsandpaper (IBoS) tests according to the procedure described herein. Thetests were conducted using 30 grit sandpaper and a 4.2 g stainless steelball having a diameter of 10 mm.

Sample sets A, D, and E are alkali aluminosilicate glasses of identicalcomposition (nominal composition about 57 mol % SiO₂, 0 mol % B₂O₃,about 17 mol % Al₂O₃, about 7% P₂O₅, about 17 mol % Na₂O, about 0.02 mol% K₂O, and about 3 mol % MgO) and thickness (0.8 mm), described in U.S.patent application Ser. No. 13/305,271, cited above. Sample set Aglasses were not ion exchanged. Sample set D glasses were ion exchangedusing the single ion exchange method described herein and had acompressive depth of layer DOL of 47 μm and a surface compressive stressCS of 885 MPa, and sample set E glasses were ion exchanged using thedual ion exchange method described herein to produce a “spike” incompressive stress CS of 865 MPa at the surface of the glass and had acompressive depth of layer DOL of 97 μm.

Sample set C consists of commercially available alkali aluminosilicateglasses (Dragontrail® Glass, manufactured by Asahi Glass Company) ofidentical composition (nominal composition about 65 mol % SiO₂, about 8mol % Al₂O₃, about 12 mol % Na₂O, about 4 mol % K₂O, about 0.3 mol %CaO, about 10 mol % MgO, and about 0.5 mol % ZrO₂) and thickness (0.8mm) The glasses of sample set B was ion exchanged using the single ionexchange method described herein, and had a compressive depth of layerDOL of 26.1 μm and a surface compressive stress CS of 795 MPa.

Sample set B consists of soda lime glass (SLG) having a thickness of 0.8mm. The sample was ion exchanged using a single step ion exchange toachieve a compressive depth of layer of 12 μm and a compressive stressof 512 MPa.

Sample thickness, depth of layer (DOL), surface compressive stress (CS),estimated average drop height at which fracture occurred, andsurvivability rates determined for a drop height of 80 cm are listed inTable 3. Heights at which fracture occurred for individual samples areplotted in FIG. 18a .

TABLE 3 Sample thickness, depth of layer (DOL), surface compressivestress (CS), and estimated average fracture height of samples subjectedto IBoS testing. Avg. Frac- % Thickness CS DOL ture Height survivalSample (mm) (MPa) (μm) (cm) at 80 cm A 0.8 — — 31 0 B 0.8 512 12 40 0 C0.8 795 26.1 47 0 D 0.8 885 47 55 ~20 E 0.8 865 97 96.3 ~70

Sample set E exhibited the greatest average fracture height (96.3 mm)and survivability rate (about 70%) for the 80 cm drop height. Sample D,having a composition and sample thickness identical to sample sets A andE but having a compressive depth of layer that was approximately half ofthat of sample E, had an average fracture/failure height of about 55 cmand survivability rate of about 20% for the 80 cm drop height. Sampleset A, which consisted of glasses having the same composition andthickness as those of sample sets D and E had a mean failure height of31 cm and a 0% survivability rate for 80 cm drop height.

Compared to ion exchanged soda lime glass (sample set B, averagefracture height of about 40 cm), ion exchanged sample set C(Dragontrail) exhibited improved inverted ball drop performance,exhibiting an average failure drop height of 47 cm. Survivability ratesfor sample sets B and C for a 80 cm drop height were both 0%.

The difference in fracture/failure heights observed for sample D andthose observed for sample E and the other ion exchanged glasses studiedillustrates the effectiveness of deep depth of layer and the stressprofile obtained by the dual ion exchange process in preventing the typeof damage to the glass experienced during field use.

Fracture/failure height also varies as a function of the thickness ofthe glass. A plot of average failure height as a function of samplethickness for sample sets B, C, D, and E is shown in FIG. 18b . As seenin FIG. 18b , the failure height of all sample groups increased withincreasing glass thickness. The samples in group E, which underwent thetwo-step ion exchange process described herein, exhibited the greatestincrease in mean failure height over the thickness range studied,followed by samples of identical composition that had undergone thesingle ion exchange process (sample set D).

While typical embodiments have been set forth for the purpose ofillustration, the foregoing description should not be deemed to be alimitation on the scope of the disclosure or appended claims.Accordingly, various modifications, adaptations, and alternatives mayoccur to one skilled in the art without departing from the spirit andscope of the present disclosure or appended claims.

The invention claimed is:
 1. A glass article having a thickness t, theglass article comprising: a compressive layer having a compressivestress CS in a range from about 500 MPa up to about 1200 MPa at asurface of the glass article, wherein: a. the compressive layer extendsfrom the surface to a depth of compression DOC, wherein DOC≧0.1·t whent<0.5 mm and DOC≧50 μm when t≧0.5 mm, and wherein the compressive layerhas a compressive stress profile; and b. the compressive stress profilecomprises: i. a first portion h extending from the surface to a depthd_(b) and having an average slope m_(b), wherein 3 μm≦d_(b)≦15 μm and−40 MPa/μm≧m_(b)≧−200 MPa/μm and wherein the first portion b is linearsuch that${{\sigma^{''}(x)}} < {{2\frac{\sigma^{\prime}(x)}{d_{b}}}}$  whereσ″(x) is the second derivative of the stress profile in the firstportion h with respect to depth x and σ′(x) is the first derivative ofthe stress profile in the first portion b with respect to depth x; ii. asecond portion c extending from d_(b) to the DOC and having an averageslope m_(c), wherein −2 MPa/μm≧m_(c)≧−8 MPa/μm and wherein the secondportion c is linear such that${{\sigma^{''}(x)}} < {{2\frac{\sigma^{\prime}(x)}{D\; O\; C}}}$where σ″(x) is the second derivative of the stress profile in the secondportion e with respect to depth x and σ′(x) is the first derivative ofthe stress profile in the second portion c with respect to depth x. 2.The glass article of claim 1, wherein −40 MPa/μm≧m_(b)≧−120 MPa/μm. 3.The glass article of claim 1, wherein thickness t is in a range fromabout 0.15 mm up to about 2.0 mm.
 4. The glass article of claim 1,wherein the glass article comprises an alkali aluminosilicate glass. 5.The glass article of claim 4, wherein the alkali aluminosilicate glassfurther comprises up to about 10 mol % Li₂O.
 6. The glass article ofclaim 1, wherein the glass article has at least about a 60% survivalrate when subjected to an inverted ball drop test with a 4.2 g stainlesssteel ball having a diameter of 10 mm from a height of about 80 cm ontoa 30 grit sandpaper disposed on the surface of the glass, wherein thesurvival rate is based on testing at least 5 samples.
 7. The glassarticle of claim 1, wherein the glass article has an equibiaxialflexural strength in a range from about 10 kgf up to about 50 kgf asdetermined by abraded ring-on-ring testing.
 8. The glass article ofclaim 1, wherein DOC≧0.11·t when t<0.5 mm and 55 μm≦DOC≦100 μm whent≧0.5 mm.
 9. The glass article of claim 4, wherein the alkalialuminosilicate glass is lithium-free.
 10. The glass article of claim 1,wherein thickness t is in a range from about 0.15 mm up to about 1.0 mm.11. The glass article of claim 1, wherein the DOC≦0.2t.
 12. The glassarticle of claim 1, wherein the second portion e is linear such that${{\sigma^{''}(x)}} < {{\frac{\sigma^{\prime}(x)}{D\; O\; C}}.}$ 13.The glass article of claim 12, wherein the first portion b is linearsuch that${{\sigma^{''}(x)}} < {{\frac{\sigma^{\prime}(x)}{d_{b}}}.}$
 14. Anelectronic device comprising the glass article of claim
 1. 15. Theelectronic device of claim 14, wherein the glass article is a coverplate, window, or display.